The midpoint formula is this:
. We have the coordinates of the midpoint and the coordinates of one of the endpoints, so our formula becomes:
. Now group the x stuff together and solve for x2 and group the y stuff together and solve for y2, like this:
. Multiply both sides by 2 to get 1=-5.5+x2. Solve for x2 to get x2 = 6.5. Do the same with the y stuff:
. Multiply both sides by 2 to get-12.5=2.75+y2 and solving for y2 gives you -15.25. So your answer is the last of your choices above. Phew!