dashed lines in intersections and graph

I was trying to repeat this graph, but only a part of it. And my goal is to make the TP curve more downward at the end. In addition, I find difficult to make the dashed lines there. Is there a way to fix it?

Some equation that might work:

TP: Q=21x+9x^2-x^3
AP: Q=21+9x-x^2 MP: 21+18x-3x^2

Graph wanted:

Code:

  documentclass{standalone}
usepackage[english,greek]{babel}
usepackage{ucs} 
usepackage[utf8x]{inputenc}
usepackage[usenames,dvipsnames]{xcolor}
usepackage{tikz}
usepackage{tkz-tab}
usepackage{cancel}
usepackage{forest}
usepackage{caption}
usepackage{latexsym}
usepackage{amssymb}
usepackage{amsmath}
usepackage{amsfonts}
usepackage{amsthm}
usepackage{pst-node}
defobj#1{makebox[1cm]{strut#1}}
usepackage{pdflscape}
usepackage{paralist}
usepackage{pst-tree,array}
usepackage{enumerate}
usepackage{graphicx}
usepackage{subcaption}
usepackage{rotating}
usepackage[usenames,dvipsnames]{xcolor}
usepackage{chngcntr}
usepackage[framemethod=tikz]{mdframed}
usepackage{wrapfig}
usepackage{color}
usepackage[explicit]{titlesec}
usepackage{appendix}
usepackage{etoolbox}
usepackage{type1cm}
usepackage{lettrine}
usepackage[stable]{footmisc}
usepackage{multicol}
usepackage{chngcntr}
usepackage{url}
usepackage{marvosym}
usepackage{tcolorbox}
usepackage[colorlinks=true,linkcolor=blue]{hyperref}
usepackage{hyperref}
tcbuselibrary{theorems}
usepackage{tkz-euclide}
usetikzlibrary{shapes.geometric}
    usepackage{tkz-fct} usetkzobj{all}
usetikzlibrary{calc,decorations.pathreplacing}
usetikzlibrary{decorations.markings}
usepackage{pgfplots}
usepackage{verbatim}
usepackage{smartdiagram}
usesmartdiagramlibrary{additions}
usepackage{tikz}
usetikzlibrary{arrows}
usetikzlibrary{intersections}








begin{document}

begin{tikzpicture}[scale=.8,>=latex,font=footnotesize,domain=0:7]
begin{scope}
  begin{scope}[yscale=-1,rotate=-90]
    draw[Red,thick,yscale=.25] plot (x,{.2*x^3-1.8*x^2+6*x})
      node[above right]{$TP$};
  end{scope}
begin{scope}[domain=1:6, rotate=0]
 draw[Green,thick,yscale=.6,name path global=MP] plot (x,{-.6*x^2+3.6*x-1.5})
    node[right]{$MP$};
draw[Blue, thick,yscale=.6,name path global=AP] plot (x,{-.2*x^2+1.8*x-.1})
    node[right]{$AP$};
end{scope}
  draw[very thick, <->](0,8)node[left]{$Q$}--(0,0)--(8,0)node[below]{$L$};
end{scope}
  draw[name intersections={of=AP and MP, by=mypoint},dashed](mypoint)--(mypoint|-0,0);

end{tikzpicture}
end{document}

documentclass[11pt, margin=1cm]{standalone}
usepackage{tikz}
usetikzlibrary{arrows.meta}
usetikzlibrary{intersections}


begin{document}

begin{tikzpicture}[scale=1, domain=0:7, every node/.style={scale=.8},
  points to/.style={thin, arrows={->[length=.5ex]},
    black!40, shorten >=5pt, shorten <=5pt},
  y={(0, .3 cm)}, samples=150]
  draw[very thin, <->] (0, 20) node[left] {$Q$} -- (0, 0)
  -- (8, 0) node[below] {$L$};
  
  draw[red, thick, name path=TP] plot (x, {-.2*x^3+1.8*x^2})
  node[right]{$TP$};
  draw[green!50!black, thick, name path=MP]
  plot (x, {-.6*x^2+3.6*x}) node[right]{$MP$};
  draw[blue, thick, name path=AP]
  plot (x, {-.2*x^2+1.8*x-.01}) node[right]{$AP$};  % needed for the point $I$

  draw[name intersections={of=AP and MP, by=I}, dashed, very thin]
  (I) -- (I|-0, 0);
  draw[dashed, very thin] (6, 0) node (MP=0){}
  -- ++(0, {-.2*6^3+1.8*6^2}) node (TPmax) {};
  path (7, 10) node[right] (label) {$TP$ max when $MP=0$}
  edge[out=-90, in=15, points to] (MP=0) edge[out=85, in=-70, points to] (TPmax);
end{tikzpicture}
end{document}