108=3x^2+15x

Solution for 108=3x^2+15x equation:

108=3x^2+15x

We move all terms to the left:

108-(3x^2+15x)=0

We get rid of parentheses

-3x^2-15x+108=0

a = -3; b = -15; c = +108;
Δ = b2-4ac
Δ = -152-4·(-3)·108
Δ = 1521
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{1521}=39$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-15)-39}{2*-3}=\frac{-24}{-6}
=+4
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-15)+39}{2*-3}=\frac{54}{-6}
=-9
$