Explanation:
the midpoint between 2 points A (xa, ya) and B (xb, yb) is
M ((xa + xb)/2, (ya + yb)/2)
(xa + xb)/2 = 10.5
(ya + yb)/2 = 9
let's say R = A
(1 + xb)/2 = 10.5
(10 + yb)/2 = 9
1 + xb = 21
xb = 20
10 + yb = 18
yb = 8
S = (20, 8)
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