Answer
$R(x)=\dfrac{L(x)}{P(x)}$
Work Step by Step
Given by the definition, the number of people in the labor force in year $x$(which is $L(x)$) divided by the civilian population in year $x$(which is $P(x)$) is equal to the participation rate in year $x$ (which is $R(x)$ ).
That is,
$R(x)=\left(\frac{L}{P}\right)(x)=\dfrac{L(x)}{P(x)}$
