x(3x-15)=900

Solution for x(3x-15)=900 equation:

x(3x-15)=900

We move all terms to the left:

x(3x-15)-(900)=0

We multiply parentheses

3x^2-15x-900=0

a = 3; b = -15; c = -900;
Δ = b2-4ac
Δ = -152-4·3·(-900)
Δ = 11025
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{11025}=105$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-15)-105}{2*3}=\frac{-90}{6}
=-15
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-15)+105}{2*3}=\frac{120}{6}
=20
$