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

so if m is the midpoint and m is (-3,-1) and one point is (-8,6) and we call point X (x,y) then

therefor

and

multiply everybody by 2
x-8=-6 and y+6=-2
add 8 to both sides or minus 6, dependin which equation
x=2 and y=-8
the coordinate of x is (2,-8)