Plotted line in tikz stops in middle of grid

The plotted line stops and doesn't go on till the border of the grid:

This is my code:

\documentclass{article}
\usepackage{tikz}
\usepackage{amsmath}

\begin{document}
\begin{tikzpicture}[baseline={(current bounding box.north)}]
  \draw[step=1cm,color=gray!20] (-3,-2) grid (6,6);
  \draw[-] (-3,0) -- (6,0) node[right] {$x$};
  \draw (0,0) node[below left] {$O$};
  \draw[-] (0,-2) -- (0,6) node[above] {$y$};
  \foreach \x in {-3, -2, -1, 1, 2, 3, 4, 5, 6}
    \draw (\x cm,1pt) -- (\x cm,-1pt) node[anchor=north] {$\x$};
  \foreach \y in {-2, -1, 1, 2, 3, 4, 5, 6}
    \draw (1pt,\y cm) -- (-1pt,\y cm) node[anchor=east] {$\y$}; 
  \draw[blue] (-2,4) node[below left] {$k$};
  \draw[black] (4,5) node[above left] {$A$}; 
  \clip (-3,-2) rectangle (6,6);
  \draw[scale=1,smooth,variable=\x,blue] plot ({\x},{(-4/3)*\x});
  \draw[scale=1,smooth,variable=\x,red] plot ({\x},{(3/4)*\x+2});
  \draw[fill=black](4,5) circle(0.5mm);
\end{tikzpicture}

\end{document}

What can I do so the line goes on the border of the grid??