A sail is cut in the shape of the triangle with corners A(1, 1, 1), B(2, 3, 3) and C(3, 2, -1)…

Step 1. The sides from $A$. $\overrightarrow{AB} = (1, 2, 2)$ and $\overrightarrow{AC} = (2, 1, -2)$.

Step 2. The angle at $A$. $\overrightarrow{AB}\cdot\overrightarrow{AC} = 2 + 2 - 4 = 0$, so the angle at $A$ is $90^\circ$.

Step 3. The side $BC$. $\overrightarrow{BC} = (1, -1, -4)$ and $\lvert\overrightarrow{BC}\rvert = \sqrt{1 + 1 + 16} = \sqrt{18} \approx 4.24$ metres.

Step 4. The angle at $B$. $\overrightarrow{BA} = (-1, -2, -2)$. The dot product with $\overrightarrow{BC}$ is $-1 + 2 + 8 = 9$. Both $\lvert\overrightarrow{BA}\rvert = 3$ and $\lvert\overrightarrow{BC}\rvert = 3\sqrt{2}$, so $\cos B = \dfrac{9}{9\sqrt{2}} = \dfrac{1}{\sqrt{2}}$ and $B = 45^\circ$.