15^2+x^2=18^2

Solution for 15^2+x^2=18^2 equation:

15^2+x^2=18^2

We move all terms to the left:

15^2+x^2-(18^2)=0

We add all the numbers together, and all the variables

x^2-99=0

a = 1; b = 0; c = -99;
Δ = b2-4ac
Δ = 02-4·1·(-99)
Δ = 396
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:

$\sqrt{\Delta}=\sqrt{396}=\sqrt{36*11}=\sqrt{36}*\sqrt{11}=6\sqrt{11}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-6\sqrt{11}}{2*1}=\frac{0-6\sqrt{11}}{2}
=-\frac{6\sqrt{11}}{2}
=-3\sqrt{11}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+6\sqrt{11}}{2*1}=\frac{0+6\sqrt{11}}{2}
=\frac{6\sqrt{11}}{2}
=3\sqrt{11}
$