Vertical angles are congruent or equal to each other. 
Set the equation: 
8x - 8 = 7x + 8 
Add 8 to both sides of the equation. 
8x - 8 + 8 = 7x + 8 + 8
8x = 7x + 16 
Subtract 7x from both sides of the equation. 
8x - 7x = 7x - 7x + 16 
x = 16 
Answer: D) 16
Checking to see if our answer is right. 
8x - 8 = 7x + 8 
8(16) - 8 = 7(16) + 8 
128 - 8 = 112 + 8 
120 = 120