Your answer would be the expression in the bottom left corner, (7 × 15) + (21 × 11) + (7 × 15).
This is because, if you think about what they're doing, they're adding the rectangle on the left (7 × 15) to the rectangle on the right (7 × 15), but then they're adding that to the rectangle that excludes the two smaller rectangles above it (21 × 11), which is incorrect.
I hope this helps!