\documentclass{standalone}
\usepackage[active,tightpage]{preview}
\usepackage{amsmath}
\usepackage{pgfplots}
\usepgfplotslibrary{fillbetween}
\usetikzlibrary{arrows.meta,intersections}
\usepackage{pgffor}
\usepackage{ifthen}

\usepackage{anyfontsize}
\renewcommand{\normalsize}{\fontsize{12pt}{14pt}\selectfont}

\pgfplotsset{compat=1.18}
\PreviewEnvironment{tikzpicture}

\begin{document}

\begin{tikzpicture}

  \begin{scope}
    \begin{axis}[
      axis lines=middle,
      xlabel={$x$},
      ylabel={$y$},
      xlabel style={xshift=10pt,yshift=-15pt},
      ylabel style={xshift=-15pt},
      xtick={0,...,6},
      ytick={0,...,11},
      grid=major,
      major grid style={gray!50}, 
      ymin=0,
      ymax=12,
      xmin=0,
      xmax=6.5,
      clip=false,
      axis on top=false,
      scale=2,
      axis equal image,
      axis line style={thick},
      width=8cm,
      legend style={at={(0.03,0.97)},anchor=north west},
    ] 
    
    \addplot [thick,black,samples=100,domain=0:6.5] { (13*x + 12)/8 };
    \legend {$y=(13x+12)/8$};
    
    \foreach \x in {0,...,6} {
      \foreach \y in {1,...,11} {
        \pgfmathparse{8 * \y <= 13 * \x + 12 ? 1 : 0}
        \ifnum\pgfmathresult=1
          \addplot[only marks, mark=*, mark options={fill=red}]
            coordinates {(\x, \y)};
        \fi
      }
    }

    \node[below left] at (axis cs: 0, 0) {$O$};
    
    \end{axis}
  \end{scope}

  \begin{scope}[
    xshift=7.5cm,
  ] 
    \begin{axis}[
      axis lines=middle,
      xlabel={$x$},
      ylabel={$y$},
      xlabel style={xshift=10pt,yshift=-15pt},
      ylabel style={xshift=-15pt},
      xtick={0,...,6},
      ytick={0,...,11},
      grid=major,
      major grid style={gray!50}, 
      ymin=0,
      ymax=12,
      xmin=0,
      xmax=6.5,
      clip=false,
      axis on top=false,
      scale=2,
      axis equal image,
      axis line style={thick},
      width=8cm,
      legend style={at={(0.03,0.97)},anchor=north west},
      legend cell align={left},
    ] 
    
    \addplot [thick,black,samples=100,domain=0:6.5] { (13 * x + 12)/8 };
    \addplot [dashed,thick,black,samples=100,domain=0:6.5] { x + 1 };

    \legend {$y=(13x+12)/8$, $y=x+1$};
    
    \foreach \x in {0,...,6} {
      \foreach \y in {1,...,11} {
        \pgfmathparse{\y <= \x + 1 ? 1 : 0}
        \ifnum\pgfmathresult=1
          \addplot[only marks, mark=*, mark options={fill=blue}]
            coordinates {(\x, \y)};
        \fi
      }
    }

    \foreach \x in {0,...,6} {
      \foreach \y in {1,...,11} {
        \pgfmathparse{((\y > \x + 1) && (8 * \y <= 13 * \x + 12)) ? 1 : 0}
        \ifnum\pgfmathresult=1
          \addplot[only marks, mark=*, mark options={fill=red}]
            coordinates {(\x, \y)};
        \fi
      }
    }

    \node[below left] at (axis cs: 0, 0) {$O$};
    
    \end{axis}
  \end{scope}

  \begin{scope}[
    xshift=15cm,
  ] 
    \begin{axis}[
      axis lines=middle,
      xlabel={$x$},
      ylabel={$y$},
      xlabel style={xshift=10pt,yshift=-15pt},
      ylabel style={xshift=-15pt},
      xtick={0,...,6},
      ytick={0,...,11},
      grid=major,
      major grid style={gray!50}, 
      ymin=0,
      ymax=12,
      xmin=0,
      xmax=6.5,
      clip=false,
      axis on top=false,
      scale=2,
      axis equal image,
      axis line style={thick},
      width=8cm,
      legend style={at={(0.03,0.97)},anchor=north west},
    ] 
    
    \addplot [thick,black,samples=100,domain=0:6.5] { (5 * x + 4)/8 };

    \legend {$y=(5x+4)/8$};
    
    \foreach \x in {0,...,6} {
      \foreach \y in {1,...,11} {
        \pgfmathparse{((8 * \y <= 5 * \x + 4)) ? 1 : 0}
        \ifnum\pgfmathresult=1
          \addplot[only marks, mark=*, mark options={fill=red}]
            coordinates {(\x, \y)};
        \fi
      }
    }

    \node[below left] at (axis cs: 0, 0) {$O$};
    
    \end{axis}
  \end{scope}

\end{tikzpicture}

\end{document}