The midpoint of rt is s(0.5, –6.25). one endpoint is t(–5.5, 2.75). what are the coordinates of r? (–4.5, –15.25) (6.5, –9.75) (–4.5, –9.75) (6.5, –15.25)

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!