Answer:
A = 5x + 12 (See explanation!)
Explanation:
So, this "polygon" is really just two rectangles combined! With that in mind, remember that the area of a rectangle is base*height. Now, apply that to the problem. We know that the total length of the bottom base is 7, and 5 is the length of the second rectangle's base, so the 1st rectangle's base must be 7 - 5 = 2. The area of the 1st rectangle is then 6*2 = 12. The second rectangle's area is 5*x. So, if the area of the polygon is the sum of the two rectangles' areas, the expression you could use is:
5*x + 6* 2 = A
Simplify/Rewrite:
A = 5x + 12
(A is area, x is the unknown length in the diagram)
Hope this helps!