I am trying to get to understand this subject but can't seem to grasp it from the books?

I have; y = X^2 + 2x - 3.

This is what I have tried so far;

(x + 2) (x - 1.5)

x = - 2 or x = 1.5

- 2 + 1.5= - 0.25

2

y = - 0.25^2 + 2 ( -0.25) - 3

( -4, -1)

I know it's wrong but can't see why?

Any help appreciated

Thanks