Question 691185
y = 5 - 3x
y=-3x+5

3x - y = 8
-y=-3x+8
y=3x-8

{{{ graph( 300, 200, -6, 5, -10, 10, -3x +5, 3x +8) }}}

These lines are perpendicular because the x multipliers 
are reciprocals (-3,+3).