x=5-y => 3(5-y)-y=3
15-3y-y=3
-4y=-12
y=3 => x=5-3=2
f"(x)=-x^2+x+2
-x^2+x+2=0
D=1+8=9
x1=-1+3=2
x2=-1-3=-4
f(2)=-8/3+2+4-3=1/3
F(-4)=-64/3+8-8-3=-73/3