Question

if M is the midpoint of AB, A(0, 4) and M(3, -2), find the coordinates of B

Answer 1

If point A is (0,4) and M is your mid point, B has to be the same distance from M as A is. So if A is __ away from M then B has to __ away from M. So for example if A is (2,2) and M is (0,0) then be has to be (-2,-2). Just do it now with your equation

Answer 2

A(x1,y1) B(x2,y2)
then their mid point
M(x3,y3)=((x1+x2)/2 , (y1+y2)/2)