midpoint formula : (x1 + x2) / 2, (y1 + y1) / 2
(4,1)....x1 = 4 and y1 = 1
(x,y)....x2 = x and y2 = y
sub
(4 + x) / 2 , (1 + y) / 2 = 3/4
(4 + x) / 2 = 3
4 + x = 3 * 2
4 + x = 6
x = 6 - 4
x = 2
(1 + y) / 2 = 4
1 + y = 4 * 2
1 + y = 8
y = 8 - 1
y = 7
ur other endpoint, R, is (2,7)
