Answer
$x \approx 9.2$
Work Step by Step
Apply the Law of Sines.
$\dfrac{\sin A}{a} =\dfrac{\sin B}{b} =\dfrac{\sin C}{c} $
Rearrange the two ratio setup to obtain:
$\dfrac{\sin A}{a} =\dfrac{\sin C}{c}\\\dfrac{\sin 48^{\circ}}{7.3} =\dfrac{\sin C}{x} \\ x=\dfrac{7.3 \sin C}{\sin 48^{\circ}} ~~~(1)$
Since, the sum of interior angles of a triangle is equal to $180^{\circ}$. Thus, we can write as:
$A+B+C =180^{\circ} \\ 48^{\circ}+21^{\circ} +C=180^{\circ} \implies C = 111^{\circ}$
Hence, the equation (1) becomes:
$x=\dfrac{7.3 \sin 111^{\circ}}{\sin 48^{\circ}} \approx 9.2$
