Question 32764
If two lines are perpendicular, the product of their slopes = -1
So first we find the slopes of both the lines
mx-y=b 
This can be re-written as 
y = mx-b   <--Slope Intercept Form
Slope in this case = m
 x+my=c
my = -x +c
{{{y = -x/m +c/m}}}
Slope in this case = {{{-1/m}}}

Now product of the 2 slopes = {{{m *(-1/m)}}}
= -1
Hence the two lines are Perpendicular