According to the given data we have the following:
Length of side a=100 ft
Lenght of side b=140 ft
Height of side h=40 ft
In order to calculate the area of this trapezoid we would have to make the following calculation:
area of this trapezoid=(a + b)/2)h
area of this trapezoid=(100+140)/2)40
area of this trapezoid=120*40
area of this trapezoid=4,800