How do you find the slope of a line perpendicular to a given line?

You have a line:

y=mx+b (slope-intercepted form)

m=slope of this line.

The slope of a line perpendicular to that given line will be: '"m'"

m'=-1/m.

For example:

y=8x+3

m=8

The solpe fo a line perpendicular to "y=8x+3" is:

m'=-1/8