Question 326287
Vertical angles equal each other.

In that case, equate both angles and solve for x.

5x = 7x - 12

5x - 7x = -12

-2x = -12

x = -12/-2

x = 6

Measure of angle 5x is 5 times 6 = 30 degrees.

Measure of angle (7x - 12) is 7 times 6 - 12 = 30 degrees.

Do you see that both angles equal 30 degrees?
  
This means they are both vertical angles because vertical angles equal each other.