solve this y2-2y-15. math?

[07]y^2-2y-15
y^2+3y-5y-15
=y(y+3)-5(y+3)
=(y+3)(y-5)

Shouldn`t it be y^2 - 2y - 15 = 0 ? ? ?

You can`t "solve" y^2 - 2y - 15, you can only express it as a product of its rational factors:

(y - 5)(y + 3)

But, if you mean:

y^2 - 2y - 15 = 0, then:

(y - 5)(y + 3) = 0, so

y = 5, or y = -3.

y^2-2y-15
y^2-5y+3y-15
y(y-5)+3(y-5)
(y-5)(y+3)

y^2 - 2y - 15
= (y + 3)(y - 5)

y² - 2y - 15 = (y - 5)(y + 3)