If you need it to be factored out, then 3a^3 + 12a^2 - 63a (That's how I'm going to write it out) can be divided out by 3a, so it turns out 3a(a^2 + 4a - 21). Next, you can further factor this out into two binomial groups using the reverse method of distribution (I forgot what it was called). So the answer is 3a(a + 7)(a - 3). You can check if it's right by distribution.
