Put the other line in point slope form...(solve for y :D )
3y=15x+9
y=5x+3, so this line has a slope of 5
For a line to be perpendicular to another the slopes have to be negative reciprocals of each other...mathematically...
s1*s2=-1
So for the other line to be perpendicular to the first:
5m=-1, m=-1/5, thus our other line (so far) is:
y=b-x/5, now we can use the point (-8,-1) to solve for the y-intercept, "b":
-1=b-(-8)/5
-1=b+8/5
-1=b+1.6
b=-2.6
So the perpendicular line that contains the point (-8,-1) is:
y=-0.2x-2.6