Answer:
b/a
Explanation:
Perpendicular lines have slopes that are opposite sign and reciprocals (flipped over).
In the equation
ax + by = c,
the slope of the line is
-a/b
If you haven't memorized this pattern yet, you can calculate it by solving ax+by=c for y.
by = -ax +c
y = -a/b x + c/b
The slope is -a/b
So a perpendicular line would be opposite sign and flipped, b/a