f(-1)+f(2)=2(-1)-3+2(2)-3

Solution for f(-1)+f(2)=2(-1)-3+2(2)-3 equation:

f(-1)+f(2)=2(-1)-3+2(2)-3

We move all terms to the left:

f(-1)+f(2)-(2(-1)-3+2(2)-3)=0

We add all the numbers together, and all the variables

f^2+f(-1)-14=0

We multiply parentheses

f^2-1f-14=0

a = 1; b = -1; c = -14;
Δ = b2-4ac
Δ = -12-4·1·(-14)
Δ = 57
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$f_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$f_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$f_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-1)-\sqrt{57}}{2*1}=\frac{1-\sqrt{57}}{2}
$
$f_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-1)+\sqrt{57}}{2*1}=\frac{1+\sqrt{57}}{2}
$