!!!!!!!!!!!!!!!!!!!!!!!!!
Y`=3x²+12x-15
3(x²+4x-5)=0
x1+x2=-4 U x1*x2=-5
x1=-5 U x2=1
+ _ +
-----------(-5)---------------(1)-----------------
max min
{21(x+1)=20(y+1)⇒21x=20y-1
{17(x+1)=20(x+y)⇒3x=17-20y
{17(y+1)=21(x+y)⇒4y=17-21x
4y=17-20y+1
24y=18
y=3/4
3x=17-15
3x=2
x=2/3
(2/3;3/4)