Answer
$A=\dfrac{21}{r}$
Work Step by Step
$A$ is inversely proportional to $r$ so the equation that relates the variables is:
$A=\dfrac{k}{r}$ where k = constant of proportionality.
If r = 3, then A = 7. Substitute these values into the equation above to have:
$A=\dfrac{k}{r}
\\7 = \dfrac{k}{3}
\\7(3)=k
\\21=k$
The constant of proportionality is $24$. Thus, the equation that relates A and r is:
$A=\dfrac{21}{r}$
