135=x^2+15

Solution for 135=x^2+15 equation:

135=x^2+15

We move all terms to the left:

135-(x^2+15)=0

We get rid of parentheses

-x^2-15+135=0

We add all the numbers together, and all the variables

-1x^2+120=0

a = -1; b = 0; c = +120;
Δ = b2-4ac
Δ = 02-4·(-1)·120
Δ = 480
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:

$\sqrt{\Delta}=\sqrt{480}=\sqrt{16*30}=\sqrt{16}*\sqrt{30}=4\sqrt{30}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-4\sqrt{30}}{2*-1}=\frac{0-4\sqrt{30}}{-2}
=-\frac{4\sqrt{30}}{-2}
=-\frac{2\sqrt{30}}{-1}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+4\sqrt{30}}{2*-1}=\frac{0+4\sqrt{30}}{-2}
=\frac{4\sqrt{30}}{-2}
=\frac{2\sqrt{30}}{-1}
$