Write the Word or Phrase That Best Completes Each Statement $p$

Write the word or phrase that best completes each statement or answers the question.
-The price $p$ of a certain product and the quantity sold $x$ obey the demand equation $p = - \frac { 2 } { 3 } x + 200,0 \leq x \leq 300$ .
Suppose that the cost $C$ of producing $x$ units is $C = \frac { \sqrt { x } } { 20 } + 800$ . Assuming that all items produced are sold, find the cost $C$ as a function of the price $p$ .