Distinct lines $\ell$ and $m$ lie in the $xy$-plane. They intersect at the origin. Point $P(1,2)$ is reflected about line $\ell$ to point $P^{\prime }$, and then $P^{\prime }$ is reflected about line $m$ to point $P^{\prime \prime }$. The equation of line $\ell$ is $3x+y=0$, and the coordinates of $P^{\prime \prime }$ are $(2,-1)$. What is the equation of line $m$?