$($b$)$ A rectangular box is partially filled with sand. The length of the box is $12$ mm $.$ The width of the box is $18$ mm $.$ I am filling the box so that the volume of sand is increasing at a rate of $520$ cubic mm per minute.

At what rate is the height of the sand changing $($in $m\frac{m}{m}in)$ when the height of the sand is $2$ mm $?$