The answer is below.
Work Step by Step
For the large shape, to get the area, you would multiply (x+3) and (x+5). To figure out what this is, one can add all of the smaller areas together. This gives $x^2 +3x +5x +15 = x^2 +8x +15$. This verifies that (x+3)(x+5) is, indeed, $x^2 +8x +15$.