$($a$)$ the density on the highway $($in veh$/$mi$)$

$v$ veh$/$mi

$($b$)$ the flow on the road $($in veh$/$h$)$

Your response is off by a multiple of ten. veh$/$h

$($c$)$ the space mean speed $($in $m\frac{i}{h})$

Your response differs from the correct answer by more than $100\%.$ mi$/$h