90=2x(x-4)

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

90=2x(x-4)

We move all terms to the left:

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


We calculate terms in parentheses: -(2x(x-4)), so:

2x(x-4)

We multiply parentheses

2x^2-8x

Back to the equation:

-(2x^2-8x)


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