First, rewrite the equation in its slope intercept form:
(slope intercept form: y = mx + b where m = slope ; b = y-axis intercept)
15x + 12y = -108
Subtract 15x from both sides:
15x + 12y - 15x = -108 - 15x
12y = -108 - 15x
Divide both sides by 12:
12y/12 = (-108 - 15x)/12
y = -5x/4 - 9
As you can see the slope of this line is -5/4
Now, two lines are perpendicular if:
m1*m2 = -1
Where:
m1 = Slope of the line 1
m2 = Slope of the line 2
m1 in this case would be -5/4
so:
-5/4* m2 = -1
Solving for m2:
m2 = -1/(-5/4) = 4/5 = 0.8