Disregard the sign. Find the mean (add the numbers, and divide by the amount of numbers there are)
2 + 12 = 14/2 = 7
Add 7 (if you are going by -2), if you are starting from 12, subtract 7
-2 + 7 = 5
5 is your midpoint.
hope this helps
The equation is: (Note: Let the two numbers be x and y)
(x + y)/2 = midpoint
hope this helps