# The slope of a line is -3. What is the slope of a line that is perpendicular to this line.?

$\frac{1}{3}$.
Lines with slopes m_1 & m_2 are $\bot$ to each other $\iff {m}_{1} \cdot {m}_{2} = - 1$.
Hence, reqd. slope $\frac{1}{3}$.