The area of a parallelogram of width W and length L is A = W*L.
The area of a trapezoid of width W and different side lengths L1 and L2 is
A = W(L1 + L2)/2.
To find the area of the trapezoid, take the average of the lengths L1 and L2 of the two parallel sides and call the result L. Then apply the formula for the area of a parallelogram, given above).
