Question 712000
 *[invoke equation_parallel_or_perpendicular "perpendicular", -3, 0, 0, -3]


the graph above is not quite good, so here is better graph to show that
{{{y=(1/3)x-3}}} is perpendicular {{{y=-3x}}}

{{{ drawing( 600, 600, -5,5, -5, 5,grid(1),circle(0,-3,0.1),graph( 600, 600, -5,5, -5, 5,-3x, (1/3)x-3)) }}}