solve quadratic formula
x-2x-15=0the first x is squared

You said that  x² - 2x - 15 = 0 .

You could factor the left side:

(x + 3) (x - 5) = 0

This equation is true if either of the factors is zero.

First factor:
x + 3 = 0
x = -3

Second factor:
x - 5 = 0
x = 5