We can do 'point' arithmetic.
M = (A+B)/2
2M = A+B
B = 2M - A
We have M(3, -4) and A(8, -5)
B = 2(3, -4) - (8, -5) = (6,-8) - (8,-5) = (-2, -3)
Check:
(A+B)/2 = ( (8+-2)/2, (-5+ -3)/2) = (3, -4) good
Answer: (-2, -3)
