Well, it is a good and interesting question
Such areas could be calculated in a single shoot and also by dividing the whole shape into other shapes and the total area would be the sum of these areas
I prefer the way of a single shoot ...
The figure represents a trapezoid:
its lower base = 21 units
its upper base = 12 units
the normal height between them = 8 units
NOW
The area of a trapezoid = [(sum of bases' lengths) ÷ 2] * height
= [(12 + 21) ÷ 2] * 8 = 132 sq units
Hope that helps