The rate of change of the unit price $p$ (in dollars) of Apex women's boots is given by $$p^{\prime}(x)=\frac{-250 x}{\left(16+x^{2}\right)^{3 / 2}}$$,where $x$ is the quantity demanded daily in units of a hundred. Find the demand function for these boots if the quantity demanded daily is 300 pairs $(x=3)$ when the unit price is $\$ 50 /$ pair.