(15+x)x=216

Solution for (15+x)x=216 equation:

(15+x)x=216

We move all terms to the left:

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

We add all the numbers together, and all the variables

(x+15)x-216=0

We multiply parentheses

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