F=-2.8+1.2x+0.8x^2

Solution for F=-2.8+1.2x+0.8x^2 equation:

=-2.8+1.2F+0.8F^2

We move all terms to the left:

-(-2.8+1.2F+0.8F^2)=0

We get rid of parentheses

-0.8F^2-1.2F+2.8=0

a = -0.8; b = -1.2; c = +2.8;
Δ = b2-4ac
Δ = -1.22-4·(-0.8)·2.8
Δ = 10.4
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{-(-1.2)-\sqrt{10.4}}{2*-0.8}=\frac{1.2-\sqrt{10.4}}{-1.6}
$
$F_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-1.2)+\sqrt{10.4}}{2*-0.8}=\frac{1.2+\sqrt{10.4}}{-1.6}
$