The base is AB = 5 ft because AB = CD. Recall that opposite sides in a parallelogram are congruent and parallel.
The height is DE = 4 ft
Area of a parallelogram is base times height. It's effectively the same formula as a rectangle (any rectangle is a parallelogram, but not the other way around).
So,
area of parallelogram = base * height = 5*4 = 20
note: the base is always perpendicular to the height
another thing to point out: the info AE = 2 ft isn't used at all. It's probably put in there to distract or to make students try to find AD using the pythagorean theorem. Knowing the length of AD will not help find the area of the parallelogram. Though now that I think about it, you could divide this parallelogram into a rectangle and two triangles. Then find the area of each small piece to add them up to get the final answer. I show what I mean in the attached image below. However, it's much easier to use the formula area = base*height.
