Mid point of (1,2) and (7,4) = ((1+7)/2, (2+4)/2) = (4, 3). (slope of the perpendicular bisector)(slope of the line joining (1,2) and (7,4)) = -1. slope of the perpendicular bisector = -1/[(2-4)/(1-7)]=-3 Since (4,3) lies on the perpendicular bisector,

the equation of the perpendicular bisector is y-3=-3(x-4), i.e. 3x+y-15=0.