20 cm, I think
The perimeter is 18 and the height is 4 cm.
4 cm x 2 = 8
We are multiplying it by two because there are two side in a rectangle that are the same height
18 - 8 = 10
We are finding the length of the rectangle
10\2 = 5
The length of the rectangle is 5
5 x 4 = 20
That is the area
