#### Answer

$x = 18$

#### Work Step by Step

We use the method of cross multiplying, which states that if $\frac{a}{b} = \frac{c}{d}$, then $ad = bc$. This result can be achieved by multiplying both sides by $cd$.
$\frac{21}{x} = \frac{\dfrac{7}{2}}{3} \rightarrow \frac{7}{2}x = 63 \rightarrow\frac{7}{2} \cdot \frac{2}{7} \cdot x = 63\cdot \frac{2}{7} \rightarrow x = 18$