midpoint between the points (x1,y1) and (x2,y2) is

so if mid is (-1,2) and an endpoint is (3,2) then
mid between (3,2) and (x2,y2) is

the x values and y values corespond
(3+x2)/2=-1
times 2
3+x2=-2
minus 3
x2=-5
other one
(2+y2)/2=2
times 2
2+y2=4
minus 2
y2=2
the other point is (-5,2)