216=x^2+15x

Solution for 216=x^2+15x equation:

216=x^2+15x

We move all terms to the left:

216-(x^2+15x)=0

We get rid of parentheses

-x^2-15x+216=0

We add all the numbers together, and all the variables

-1x^2-15x+216=0

a = -1; b = -15; c = +216;
Δ = b2-4ac
Δ = -152-4·(-1)·216
Δ = 1089
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{1089}=33$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-15)-33}{2*-1}=\frac{-18}{-2}
=+9
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-15)+33}{2*-1}=\frac{48}{-2}
=-24
$