Answer
$A = k \cdot \dfrac{x^2}{t^3}$
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.
$A$ is given to be directly proportional to $x^2$ and inversely proportional to $t^3$ therefore the equation that expresses the given statement is:
$A = k \cdot \dfrac{x^2}{t^3}$ where k = constant of proportionality