When I factor
You are wrong.
What is the greatest common factor in the 2 terms?
The greatest common factor is x^2. So factor x^2 out from both terms.
(x^2) (x - 8)
x^3 - 8x^2 can factor out to
(x^2) * (x - 8)
(x^2 - 5x)(x-3) would equal x^3 - 8x^2 -+ 15x