The third angle is $\angle D = 180^\circ - 45^\circ - 73^\circ = 62^\circ$.

Side $FE = 13$ is opposite $\angle D$, and $DE = x$ is opposite $\angle F = 73^\circ$. By the Law of Sines: $\frac{\sin 62^\circ}{13} = \frac{\sin 73^\circ}{x}$