to find the perpendicular bisector, you have to find the midpoint of mn
which is

<-- midpoint of mn
next you find the slope of mn which is

<- this is the slope
to find the perpendicular slope, you take the negative recipricol which is
-2
so now we know the perpendicular bisector line has a slope of -2 and passes through (-4, -2)
so
y-(-2) = -2(x-(-4))