Answer
$A=\dfrac{18x}{t}$
Work Step by Step
$A$ is directly proportional to $x$ and inversely proportional to $t$ so the equation that relates the variables is:
$A=k \cdot \frac{x}{t}$ where k = constant of proportionality.
If x = 7 and t = 3, then A = 42. Substitute these values into the equation above to have:
$A=k \cdot \frac{x}{t}
\\42 = k \cdot \frac{7}{3}
\\42=\frac{7k}{3}
\\3(42)=7k
\\126=7k
\\\frac{126}{7}=k
\\18=k$
The constant of proportionality is $18$. Thus, the equation that relates A, x and t is:
$A=18 \cdot \frac{x}{t}
\\A=\frac{18x}{t}$