#### Answer

$R = k \cdot \dfrac{P^2t^2}{b^3}$
where k = constant of proportionality

#### Work Step by Step

RECALL:
(i) If the quantities (or variables) $x$ and $y$ are related by the equation $y=kx$, then $y$ is directly proportional to $x$, where $k$ is the constant of proportionality.
(ii) If the quantities (or variables) $x$ and $y$ are related by the equation $y=\dfrac{k}{x}$, then $y$ is inversely proportional to $x$, where $k$ is the constant of proportionality.
$R$ is given to be directly proportional to the product of $P^2$ and $t^2$ and inversely proportional to $b^3$ therefore the equation that expresses the given statement is:
$R = k \cdot \dfrac{P^2t^2}{b^3}$
where k = constant of proportionality