Answer
See below
Work Step by Step
Use the law of sines:
$$\frac{a}{\sin A}=\frac{b}{\sin B}=\frac{c}{\sin C}$$
First we obtain: $\frac{a}{\sin A}=\frac{b}{\sin B}\\\sin B=\frac{a}{\sin A}\times b=\frac{11\sin73^\circ}{18}\approx 0.5844\\B\approx\arcsin (\sin B)=35.76$
We know the sum of angles of a triangle is $180^\circ$:
$C=180^\circ-B-A\\C=180^\circ-35.76^\circ-73^\circ=71.24^\circ$
Then: $c^2=a^2+b^2-2ab\cos C\\c^2=18^2+11^2-2\times18\times11\cos 71.24^\circ\\c^2\approx317.644\\c\approx17.8$
