Ok, we are going to use the midpoint formula
M = ( (x1 + x2) / 2 , (y1 + y2) / 2 )
(-7,-21)=((x1-13)/2 , (y1-15)/2 )
Break up this formula into two equations.
(x1-13)/2=-7 and (y1-15)/2=-21
Solve for x1 and y1 from the equations. So:
x1=(-7*2)+13
x1=(-14)+13=-1
y1=(-21*2)+15=(-42)+15=-27
So the other endpoint is (-1, -27).
