EXPLANATION
Let's see the facts:
First wall Area= 4x^2 + 12x + 9
Second wall Area = 36x^2 - 12x + 1
So, we need to add the first polynomial with the second one.
(4x^2 + 12x + 9) + (36x^2 - 12x + 1)
Rearranging similar terms:
4x^2 + 36x^2 + 12x -12x + 9 + 1
Adding similar terms:
40x^2 + 10 = Total Area of the two walls