I think you will like math better if you understand what you are doing, rather than plugging numbers into formulas.
The slope of a line tells you how fast you go up or down as you move left to right along the line. The given slope of -4 means you go down 4 units every time you move one unit to the right.
So think about walking left to right between the two given points.
The point (-2,r) is to the left of (12,10), so you are starting at x=-2 and moving to x=12. How far do you move in the x direction? From -2 to +12, a change of 14.
The slope is -4, so you go down 4 units for each change of 1 in x. So when x changes by 14, y changes by 14(-4) = -56.
So you went down 56 units from a y value of "r" to a y value of 10. So what must the value of "r" be? 10+56 = 66.
When you get an answer for a problem like this in this way, you are doing the same arithmetic you would be doing if you plugged the numbers into the slope formula. But you will have a better understanding of what you are doing... and most likely you will be much less likely to get a wrong answer.