Answer: 18 feet
=====================================================
How to get the answer:
We first need to compute the area of the trapezoid
area of trapezoid = height*(base1+base2)/2
A = h*(b1+b2)/2
A = 9*(12+24)/2
A = 9*(36/2)
A = 9*18
A = 162
The trapezoid has an area of 162 square feet.
---------
The rectangle also has an area of 162 sq ft.
area of rectangle = length*width
A = L*W
A/W = L
L = A/W
L = 162/9
L = 18
The length of the rectangle is 18 feet.
---------
Side note: both rectangle and trapezoid have the same height (9 feet). Notice how the average of the 12 ft and 24 ft sides leads to 18 ft, which is exactly the value of L. It's not a coincidence these two facts line up.
