x-2x-15=0

the first x is squared

$$x^2-2x-15=0\\ x^2-5x+3x-15=0\\ x(x-5)+3(x-5)=0\\ (x+3)(x-5)=0\\ x=-3 \vee x=5$$

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

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