import solids;
currentprojection=orthographic(5,3,4);

size(7.5cm);

real a=6,b=4,c=3;

draw(box(O,(a,b,c)),blue);

limits(O,X+Y+Z);
xaxis3(scale(1.5)*Label("$x$",1),green,Arrow3);
yaxis3(scale(1.5)*Label("$y$",1),green,Arrow3);
zaxis3(scale(1.5)*Label("$z$",1),green,Arrow3);

triple pA=(a,b,0), pB=(0,b,0), pC=(0,.9b,c), pM=(a,0,c);
transform3 sym=reflect(pA,pB,pC);
triple imM=sym*pM, pN=(pM+imM)/2;

draw(surface(pA--pB--(pB+pN-pA)--pN--cycle),palegray+opacity(.5));
label("$A$",pA,X);
label("$B$",pB,Y);
dot("$C$",pC,Z);
dot("$M$",pM,Z-Y,red);

dot(pN,gray);
dot("$M'$",imM,Y-Z,green);
draw(pM--imM,1bp+dashed);