This is really a right triangles question. To see why, a visual aid might help. Here’s the x = 2 line and the x = 14 line:

The straight line distance between the lines is 12. 

If we keep (2, 0) as one of our points, and move up or down on the x = 14 line, we can see the triangle form.

We know the hypotenuse must be 20, and we know one leg is 12, so we just need to solve for the other leg.

12^2 + b^2 = 20^2
144 + b^2 = 400
b^2 = 256
b = 16

So the vertical leg must be 16 units long for the hypotenuse to be 20 units long.

(Shortcut opportunity: 12-16-20 is a Pythagorean triple—a bigger brother of the 3-4-5!)