90=5x*2x

Solution for 90=5x*2x equation:

90=5x*2x

We move all terms to the left:

90-(5x*2x)=0

We add all the numbers together, and all the variables

-(+5x*2x)+90=0

We get rid of parentheses

-5x*2x+90=0

Wy multiply elements

-10x^2+90=0

a = -10; b = 0; c = +90;
Δ = b2-4ac
Δ = 02-4·(-10)·90
Δ = 3600
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{3600}=60$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-60}{2*-10}=\frac{-60}{-20}
=+3
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+60}{2*-10}=\frac{60}{-20}
=-3
$