put it into slope intercept form y=mx+b where m is slope
0=-15-7x+3y
subtract 3y from both sides
-3y=-7x-15
multiply both sides by -1
3y=7x+15
divide both sdies by 3
y=7/3x+5
slope is 7/3
to find a line perpendicular to it, the slope of the other line, when multiplied by this line, must equal -1, exg
1/2 and -2/1 are perpenicular
-1 and 1 are perpendicular
therefor the line perpendicular to it has a slope of -3/7, exg y=-3/7+3