Given a tolal$-$revenue function $R(x)=900\sqrt[\phantom{2}]{x^2-0\mathrm{.}1x}$ and a total$-$cost function $C(x)=2000(x^2+2{)}^{\frac{1}{3}}+800,$ both in thousands of dollars, find the rate at which tolal profit is changing when $x$ items have been produced and sold.

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