(3x-15)(2x)=90

Solution for (3x-15)(2x)=90 equation:

(3x-15)(2x)=90

We move all terms to the left:

(3x-15)(2x)-(90)=0

We multiply parentheses

6x^2-30x-90=0

a = 6; b = -30; c = -90;
Δ = b2-4ac
Δ = -302-4·6·(-90)
Δ = 3060
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{3060}=\sqrt{36*85}=\sqrt{36}*\sqrt{85}=6\sqrt{85}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-30)-6\sqrt{85}}{2*6}=\frac{30-6\sqrt{85}}{12}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-30)+6\sqrt{85}}{2*6}=\frac{30+6\sqrt{85}}{12}
$