How do you find the slope of the line perpendicular to y = -3x + 9?

If a line has a slope of $m$
any perpendicular to it has a slope of $- \frac{1}{m}$
$y = - 3 x + 9$
is an equation in the slope y-intercept form where the slope is $- 3$
Therefore the slope of any line perpendicular to this line is $\frac{1}{3}$