Contextual Applications of Differentiation · Easy problem

A company's revenue from selling $x$ units is $R(x)=80x$ dollars, and its cost is $C(x)=0.5x^2+20x+200$ dollars, so its profit is $P(x)=R(x)-C(x)$. Use the marginal profit at $x=50$ to estimate the extra profit, in dollars, earned from making and selling the 51st unit.