x^2+2x-15 (all over) x-3 equals 3.

(x-3) * x^2+2x-15 (all over) x-3 equals 3 *(x-3)

x^2+2x-15=3x-9

x^2-x-6=0

(x-3)(x+2)=0

x-3=0 or x+2=0

x={3, -2}

I did something wrong here but I'm not sure what I did that is wrong. What is my error? Or is it the solution?

Nothing wrong in your steps,

except at the end when you have
x = 3 or x = -2, we can't allow x = 3, since we then would have divided the original by zero, which we can't do
so the only answer is x = -2

Had you factored like in the previous ....
(x+5)(x-3)/(x-3) = 3
x+5 = 3
x = -2