Area of a compound figure
A compound figure can generally be divided into smaller figures and its area is the sum of all the smaller areas.
The image provided can be divided into a triangle and a rectangle (see image below):
The total area is the sum of the area of the triangle and the area of the rectangle.
Where B=20 is the base of the triangle and H=10, thus:
The area of the rectangle is
Ar = W*L, where W=20, L=40
Ar = 20*40 = 800
The total area is 100 + 800 = 900
Total area= 900