The perimeter of a trapezoid is 39a - 7. Three sides have the following lengths: 9a, 5a + 1, and 17a - 6. What is the length of the fourth side?

A. 12

B. a - 7

C. 8a - 2

D. 31a - 5

We know the perimeter is the total sum of all sides of the shape, we can use this to form an equation: the perimeter=1st side + 2nd side + 3rd side + 4th side 39a-7=9a + (5a+1) + (17a-6) + 4th side.

Collect like terms to get 39a-7=31a-5+4th side

If we move everything but the 4th side over to the left we can solve

C. 8a-2=4th side is the correct answer