Differential equations — practice question

In one country, the government levies tax on every litre of petrol bought by motorists. The annual revenue is $R$ million dollars when the tax rate is $x$ dollars per litre. The relationship between $R$ and $x$ is represented by the differential equation $\frac{dR}{dx} = R\left(\frac{1}{x} - 0.57\right)$, with $R$ and $x$ treated as continuous variables. When $x = 0.5$, $R = 16.8$.