400=120x-5x^2

Solution for 400=120x-5x^2 equation:

400=120x-5x^2

We move all terms to the left:

400-(120x-5x^2)=0

We get rid of parentheses

5x^2-120x+400=0

a = 5; b = -120; c = +400;
Δ = b2-4ac
Δ = -1202-4·5·400
Δ = 6400
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{6400}=80$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-120)-80}{2*5}=\frac{40}{10}
=4
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-120)+80}{2*5}=\frac{200}{10}
=20
$