Byron S. answered • 10/02/14

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Since you know the unit price, you can find the demand quantity by substituting p=63 and solving for x.

You're then looking for the rate of change of the quantity, which means a derivative. Taking the implicit derivative with respect to t, you get

dp/dt + d (x^2) / dt = 0

dp/dt + 2x dx/dt = 0

dp/dt is given in the problem, and you solved for x earlier, so you can now solve this for dx/dt.