Chapter 3 - Section 3.2 - Problem Solving and Business Applications Using Systems of Equations - Exercise Set - Page 207: 55

$P(x)=200,000-50x$

Let $x$ be the manufacturing cost.The fixed cost of a phone is $200,000$. During each bicycle it costs $250$. Selling price per phone = 300 Thus, the revenue function R(x) will be $R(x)=300x$ and Cost function $C(x)=200,000+250x$ Amount of profit function $P(x)=R(x)-C(x)$ or, $P(x)=300x-(200,000+250x)$ Hence, $P(x)=200,000-50x$