Answer:
9
Step-by-step explanation:
You know that the the center line is equal to half the sum of the legs, because it is the midline. You add the legs together (7x+13) and (5x+7), and you get 12x+20. Divide that by 2 and you get 6x + 10. The midline is 8x-8, so you set up an equation, and get 6x+10 = 8x-8. That simplifies to 18=2x. Divide both sides by 2 and you get x = 9
