Area of a trapezoid
We know that a trapezoid is a quadrilateral with a pair of parallel sides:
it has a height:
In order to find its area we follow the equation:

Step 1: adding the parallel sides
In this case the parallel sides are:
a = 17 ft
b = 39 ft
a + b = 56 ft
Step 2: dividing the result by two
We have that

Step 3: multiplying by the height
In this case the height is
h = 16 ft
then

Then, in this case the area is:
area = 448 ft²