The picture can be divided into 2 same triangles and one rectangular.
The triangle areas is {1/2 *base * height}*2 (because 2 same triangles)
and you need to sum rectangular area (width * base).
The result will be area of 2 triangle +area of rectangular.
=2(1/2*base of triangle* height)+ (base of rectangular * height)
= (base of triangle* height)+(base of rectangular * height)
=(base of triangle +base of rectangular)*height
b1 is top length
b2 is bottom length, and so
Base of triangle =(b1-b2/2)
Base of rectangular =b2
Height =h
Thus A ={(b1-b2/2)+b2 }*h
A= [(b1+b2)/2]*h