jmc

We find the equation of $m$ first. Since it is perpendicular to $l$, its slope must be $-1\times (4{)}^{-1}$. Thus $a=-1/4$. Since $m$ also passes through the point $(2,1)$, we can find the equation of line $m$ by substituting 2 for $x$ and $1$ for $y$ in $m$'s point-slope form: $1=2\times -\frac{1}{4}+t$, where $(0,t)$ is the $y$-intercept of $m$. $t=\frac{3}{2}$. Thus, at $x=6$, the equation of line $m$ has y value $-6\times \frac{1}{4}+\frac{3}{2}=\boxed{0}$.