Chapter 6 - Systems of Equations and Inequalities - 6-2 Solving Systems Using Substitution - Mixed Review - Page 373: 52

$-\frac{1}{3}$

For two lines to be perpendicular, their slopes must be opposite reciprocals, so we are looking for the opposite reciprocal of the slope of the given equation. The given equation is $y=3x$. Since the $3$ is the coefficient of $x$ in this equation, the slope is $3$. The opposite reciprocal of $3$ is $-\frac{1}{3}$.