X(x+15)=18

Solution for X(x+15)=18 equation:

X(X+15)=18

We move all terms to the left:

X(X+15)-(18)=0

We multiply parentheses

X^2+15X-18=0

a = 1; b = 15; c = -18;
Δ = b2-4ac
Δ = 152-4·1·(-18)
Δ = 297
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:

$\sqrt{\Delta}=\sqrt{297}=\sqrt{9*33}=\sqrt{9}*\sqrt{33}=3\sqrt{33}$
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(15)-3\sqrt{33}}{2*1}=\frac{-15-3\sqrt{33}}{2}
$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(15)+3\sqrt{33}}{2*1}=\frac{-15+3\sqrt{33}}{2}
$