Question 74540
X-7Y=21, -7Y=-X+21, Y=-X/-7+21/-7, Y=X/7-3
7X+Y=7, Y=-7X+7
{{{ graph( 200, 200, -6, 5, -6, 5, y = x/7 -3, y = -7x +7) }}} (graph 200x200 pixels, x from -6 to 5, y from -6 to 5, of TWO functions y = x/7 -3 and y = -7x +7). 
they look perpendicular to me AND the slopes are negative recipricals of each other. (definition of parallel lines).