The per-unit profit of a product is given by $P=600-5x$, where $x$ is the number of units sold. What is the expression for marginal revenue?

Revenue $R(x)$ is calculated as the product of the price (or per-unit profit in this context) and the number of units sold:
$$R(x)=(\text{Price per unit})\times x$$
$$R(x)=(600-5x)\cdot x=600x-5x^2$$
Marginal Revenue ($MR$) is the derivative of the Revenue function with respect to $x$:
$$MR=\frac{dR}{dx}=\frac{d}{dx}(600x-5x^2)$$
$$MR=600-10x$$