Demand for a certain kind of SUV obeys the following equation,

$D(x,y)=25,000-\frac{1}{4}\sqrt[\phantom{2}]{x}-12(0\mathrm{.}7y-10{)}^{\frac{3}{2}}$

where $x$ is the price per car in dollars, $y$ is the cost of gasoline per litre, and $D$ is the number of cars.

Suppose that the price of the car and the price of gasoline $t$ years from now obey the following equations:

$x=61,200+400t,y=121+14\sqrt[\phantom{2}]{t}$

What will the rate of change of the demand be $($with respect to time$)3$ years from now?