Question 1110141
{{{system(8x+5y=120, and, 5x-8y=-80)}}}


The slopes are {{{-8/5}}} and {{{5/8}}}.
You can tell by looking at the equations, or if necessary, solving for y in terms of x, and adjust to slope-intercept form.


THIS should tell you something:   {{{-(8/5)*(5/8)=-1}}}.