SOLUTION: I'm not sure if I'm in the right section, but I would really appreciate help with this problem it says prove that mx-y=b and x+my=c are perpendicular.

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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

Slope in this case =
Now product of the 2 slopes =
= -1
Hence the two lines are Perpendicular