Answer:
The coordinates of the other endpoint are (2b-g,2m-r)
Explanation:
Let
R (x,y) ----> the other endpoint of the segment
we have
M(b,m) and Q(g,r)
we know that
The formula to calculate the midpoint between two points is equal to

In this problem the midpoint between Q and R is equal to the point M
substitute

---->

---->
therefore
The coordinates of the other endpoint are (2b-g,2m-r)