It's simple. The base of the rectangle is equal to the base of the big triangle. The height of the rettangle is equal to the base of the small triangle.
First you can calculate the area of the rectangle.
Ar = b*h = 5*8 = 40 in^2
Then you calculate the area of the two triangles
Small triangle area = (b*h)/2 = (6*5)/2 = 30/2 = 15 in^2
Big triangle area = (b*h)/2 = (8*7)/2 = 56/2 = 28 in^2
