Because the picture shows that there's a 90 degree angle in the bottom. We know that the bottom triangle is a 45-45-90 triangle. So the length of one side is 18 because a 45 45 90 triangle has two sides that are equivalent (x) and one side that is x. So now that we know the length of one side is 18, we know the lengths of all 4 sides. Adding them up ( 18 + 18 + 18 + 18) gives 72, which is the perimeter.
