The coordinates of B would be (2, -4)
In order to find this, we need to know that the value of M's points will always be the average of A and B's points. This is because it is the midpoint. Therefore we can use the following formula.
Value of x
(A + B)/2 = M
(-8 + B)/2 = -3
-8 + B = -6
B = 2
Then we can do the same for the y values
Value of y
(A + B)/2 = M
(-2 + B)/2 = -3
-2 + B = -6
B = 4