Question

M(4,2) is the midpoint of RS. The coordinates of S are (6, 1). what are the coordinates of R?

Answer 1

Remark
You are using the midpoint formula. Instead of finding the midpoint, you are looking for one of the points, so you have to rearrange the formula a little bit.

Givens
Midpoint (4,2)
One endpoint (6,1)

Object
Find the other endpoint.

Formula
m(x,y) = (x1 + x2)/2, (y1 + y2)/2)

Solution
Find the x value
4 = (6 + x2)/2    Multiply both sides by 2
4*2 = 6 + x2      Subtract 6 from both sides.
8 - 6 = x2
x2 = 2

Find the y value
2 = (1 + y2)/2    Multiply by 2
4 = 1 + y2          Subtract 1 from both sides.
4 - 1 = y2  
y2 = 3

Conclusion
R(x,y) = (2,3)