1. We start by looking at the 12. it goes the whole length of both sides of your figure. So, we can skip adding those 3's, and add two 12's.
2. So far, you have got 24 ft. Now, we still must add the width in so you add 6 more to that 24 (6 because 3 ft. on each side-you add). Your final answer is 30 ft. Hope this helps :)
