Answer:
In the first box: 24a
In the second box: +12
In the third box: 24a + 12
Explanation:
For the first box, you're multiplying 3 times 8a. Since we don't know a, we leave it alone and just multiply the number next to it. So 3 x 8 = 24...3 x 8a = 24a
3 times a positive 4 is a positive 12 (this is kind of a weird way to write it, but area models are weird, imo.
It's just trying to show that you multiply the 3 by every part of what's inside the parentheses.