The midpoint of (AB) is M = (1,3). One endpoint is A =(8,-2). Find the coordinates of the other endpoint B.

Answer 1

1 = (x+8)/2
2 = x+8
x = -6

3 =(y-2)/2
6 = y-2
y = 8

so (-6,8)

Answer 2

MP = (1,3), A = (8,2), B = (x2,y2)
Midpoint x = 1/2(x1 + x2)
Midpoint y = 1/2(y1 + y2)

1 = 1/2(8 + x2)
2 = 8 + x2
x2 = -6

3 = 1/2(-2 + y2)
6 = -2 + y2
y2 = 8

B = (x2,y2) = (-6,8)