The other end point would be (13, -20).
In order to solve these, you must look at the formula for finding the midpoint.
midpoint(x) = (x1 + x2)/2
midpoint(y) = (y1 + y2)/2
Using these two formulas, we can solve for the second point. Let's start with the x value.
midpoint(x) = (x1 + x2)/2 ----> plug in known values
6 = (-1 + x2)/2 ----multiply both sides by 2
12 = -1 + x2 ----> add 1
13 = x2
Now do the same for y.
midpoint(y) = (y1 + y2)/2 ----> plug in known values
-8 = (4 + y2)/2 ----> multiply both sides by 2
-16 = 4 + y2 ----> subtract 4 from both sides
-20 = y2
Given these two point, we now have the ordered pair of (13, -20) for the other end point.