From what I am seeing, a "short" is a 1, and a "long" is an x.
So, you look at the side lengths.
The longer side length has two "longs" and three "shorts", so that is 2x + 3.
The shorter one has one "long" and one "short", which is x + 1.
Because those two are multiplied to create an area, the side lengths are multiplied. That gives us the answer you circled.