An easy way to find an endpoint when the midpoint is given is to simply count the distance from the given endpoint to the midpoint, and go that distance from the midpoint in the opposite direction of the other endpoint. Using your problem as an example:
The y value doesn't change, so we only need to worry about x.
From 1 to -4, you go left 5 (-4 - 1 = -5). So, do that again, and you get -9.
Therefore, the coordinates of the other endpoint are (-9, -2), or B.
Hope this helps! :)
