In order to calculate the length of NB, let's use the cosine relation of the angle 30°.
The cosine relation is the length of the adjacent side to the angle over the length of the hypotenuse.
So we have:
[tex]\begin{gathered} \cos (30\degree)=\frac{NB}{BL} \\ \frac{\sqrt[]{3}}{2}=\frac{NB}{18} \\ 2\cdot NB=18\sqrt[]{3} \\ NB=9\sqrt[]{3} \end{gathered}[/tex]
