So, there are two easy ways to solve for the missing coordinate. The first would be to graph the point with known coordinates and use rise/run to get to the point with the missing coordinate. Using this method, you would find that if x = -4, then r = 6.
Alternatively, you could write these variables into point-slope form so that you could solve for r like so:
y - y0 = m • (x - x0)
Where y is actually r. You can simply add y0 to the other side, plug in your known variables and get the following equation:
r = 2 • ( -4 - (-6) ) + 2 = 6