Hey Victoria!
For some stuff like this, you can simplify it then you would factorize it. However, we can't do that for this one because this one doesn't have any like terms.
So, we just need to use our intelligence to find out the factorization form.
So we have:
x^3 + 3x^2
= x^2(x + 3)
Best of luck with your studies, Victoria.