$(30$ pts$)$ Let's assume the air in a city to be represented as a box that is $121km$ wide, $121km$ long, and $1\mathrm{.}0km$ high. A clean air enters at one side at a speed of $4\mathrm{.}50\frac{m}{s},$ and an air pollutant with a rate constant of $k=\frac{0\mathrm{.}17}{h}r$ is being emitted into the box at a rate of $11\mathrm{.}5k\frac{g}{s}.$

$($a$)(15$ pts$)$ Determine the concentration in a state of equilibrium if the air is thoroughly mixed.

$($b$)(15$ pts$)$ When the equilibrium concentration was achieved, the windspeed suddenly drops to $2\mathrm{.}50\frac{m}{s}.$ What would be the concentration of pollutants one hour later of this sudden drop?