2x(x-4)=90

Solution for 2x(x-4)=90 equation:

2x(x-4)=90

We move all terms to the left:

2x(x-4)-(90)=0

We multiply parentheses

2x^2-8x-90=0

a = 2; b = -8; c = -90;
Δ = b2-4ac
Δ = -82-4·2·(-90)
Δ = 784
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{784}=28$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-8)-28}{2*2}=\frac{-20}{4}
=-5
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-8)+28}{2*2}=\frac{36}{4}
=9
$