I say that the answer is 7 bc it’s -8+15
The original area of a face would be a^2. Now that you added b to the edge, the new area of each face would be (a+b)^2. To find how much the are increased, subtract a^2 from (a+b)^2.
So the answer is b(2a+b)
T chart but thats all i can think of