Given a total$-$revenue function $R(x)=600\sqrt[\phantom{2}]{x^2-0\mathrm{.}3x}$ and a total$-$cost function $C(x)=1400(x^{2}3{)}^{\frac{1}{3}}900,$ both in thousands of dollars, find the rate at which total profit is changing when $x$ items have been produced and sold.

$P^{\text{'}}(x)=$