90=5x^2-15x

Solution for 90=5x^2-15x equation:

90=5x^2-15x

We move all terms to the left:

90-(5x^2-15x)=0

We get rid of parentheses

-5x^2+15x+90=0

a = -5; b = 15; c = +90;
Δ = b2-4ac
Δ = 152-4·(-5)·90
Δ = 2025
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{2025}=45$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(15)-45}{2*-5}=\frac{-60}{-10}
=+6
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(15)+45}{2*-5}=\frac{30}{-10}
=-3
$