Explanation:
what I understood from your scrambled problem definition :
A = (-4, 8)
M = midpoint between A and B = (1, 7.5)
we need to find B.
the midpoint between 2 points C and D is in general :
(Mx, My) = ((Cx + Dx)/2, (Cy + Dy)/2)
so, in our case that is
(1, 7.5) = ((-4 + Bx)/2, (8 + By)/2)
that gives us 2 equations :
1 = (-4 + Bx)/2
7.5 = (8 + By)/2
2 = -4 + Bx
Bx = 6
15 = 8 + By
By = 7
B = (6, 7)