Question

How do you find what x is?

(-2,8) and (x,12)

Answer 1

Unless you know the slope, you can not.
What is the rest of the problem?

Answer 2

They are ordered pairs and are for the same direct variation.

Answer 3

Oh, you need to know it is direct variation, That gives you the slope because the line goes through two known points, (0,0) and (-2,8).
slope = (8-0)/(-2-0) = -4
y = -4 x
12 = -4 x
x = -3