The measure of two vertical angles are 3x and 6 - 12. Find a.
The measure of vertical angles are always equal to each other. So, we can set up an equation:
3x = 6 - 12
Simplifying the right side of the equation, we have:
3x = -6
Dividing both sides by 3, we get:
x = -2
Therefore, a = -2.