Y=-0.4x2+8x

Solution for Y=-0.4x2+8x equation:

=-0.4Y^2+8Y

We move all terms to the left:

-(-0.4Y^2+8Y)=0

We get rid of parentheses

0.4Y^2-8Y=0

a = 0.4; b = -8; c = 0;
Δ = b2-4ac
Δ = -82-4·0.4·0
Δ = 64
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$Y_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$Y_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{64}=8$
$Y_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-8)-8}{2*0.4}=\frac{0}{0.8}
=0
$
$Y_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-8)+8}{2*0.4}=\frac{16}{0.8}
=20
$