-3 10 setxrange
-3 6 setyrange
10 setfontsize 
18 setxunit
/arrowscale {.7 dup} def
/A {4 0} def
/B {4 4} def
/C {0 4} def
/F {-2 2} def
/G {-2 -2} def

-.8 1.6 times2

quadrillage
tracerepere
marks

A F C parallelopoint /H defpoint
A C milieu /I defpoint
6 F B A paral xdpoint /M defpoint

[A F C H] polygone
[A B C F G H I M] {times2} plot
A G droite
F B droite

.7 setlinewidth
pointilles
[C G] ligne

.5 setlinewidth
2 horizontale droite
{neg 4 add} courbe

#tex# $y = 2$
9 2 uctexlabel
#tex# $y = -x+4$
6 -2 brtexlabel
setTimesItalic
   (A) A drtext
   (B) B ultext
   (C) C urtext
   (F) F ultext
   (G) G ultext
   (H) H urtext
   (I) I urtext
   (M) M ultext