How do you factor 9a^3 + 15a^2

- - Look at both terms.

Notice that they can both be divided by 3.

Factor out 3.

3 (3a³ + 5a²)

- - Look at the terms inside the parentheses.

Notice that they can both be divided by a².

Factor the a² out, to the outside of the parentheses.

3 a² (3a + 5)

No factor left that's common to both terms

inside the parentheses.

So it's now completely factored.