MISSING COORDINATE Midpoint formula..plz helpHW...?

Question

M(6, -6) is the midpoint of RS(line segment). If S has coordinates of (15,15) find the coordinates of R...

Answer 1

R is (-3,-27)

Answer 2

Ok the x coordinate difference is 9, which means that the x-coordinate for R is 6-9 which is -3.

The y-coordinate diference is 21 which means that the y-coordinate for R is -6 - 21 which is -27.

This means that point R is (-3, - 27)  

Answer 3

(x1+x2)/2, (y1+y2)/2

(15+x2)/2 = 6

x2 = -3

(15+y2)/2 = -6

y2 = -27

Therefore the answer is (-3, -27)