If the midpoint of line segment has a coordinates (1,-5) and A= (-8,7)

Question

then which coordinates of B?

Answer 1

-8 to +1 is nine in x direction
so go another 9
1 + 9 = 10, the x coordinate of B
same idea with y
+7 to -5 is down 12
go down another 12 to -17
so
(10 , -17 )