The midpoint formula is this:[tex]( x_{midpt} , y_{midpt})=( \frac{ x_{1} + x_{2} }{2} , \frac{ y_{1}+ y_{2} }{2} )[/tex]. We have the coordinates of the midpoint and the coordinates of one of the endpoints, so our formula becomes:[tex](.5, -6.25)=( \frac{-5.5+ x_{2} }{2}, \frac{2.75+ y_{2} }{2}) [/tex]. Now group the x stuff together and solve for x2 and group the y stuff together and solve for y2, like this:[tex].5= \frac{-5.5+ x_{2} }{2} [/tex]. 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:[tex]-6.25= \frac{2.75+ y_{2} }{2} [/tex]. 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!
