I am not sure at the moment if Chris and I have interpreted the question the same.
(This diagram is drawn to scale)
Here is my interpretation.
$$\\
Let length BC = x
Now I will use the cosine rule:
$$\boxed{cosA=\frac{b^2+c^2-a^2}{2bc}}\\\\
so\\\\
cos\theta=\frac{8^2+16^2-x^2}{2\times8\times16}\quad and \quad cos\theta=\frac{\sqrt{336}^2+12^2-x^2}{2\times \sqrt{336}\times12}\\\\
Hence\\\\
\frac{8^2+16^2-x^2}{2\times8\times16}=\frac{\sqrt{336}^2+12^2-x^2}{2\times \sqrt{336}\times12}\\\\
\frac{320-x^2}{128}=\frac{480-x^2}{ 12\times 4\sqrt{21}}\\\\
\frac{320-x^2}{128}=\frac{480-x^2}{ 12\times 4\sqrt{21}}\\\\
\frac{320-x^2}{8}=\frac{480-x^2}{ 3\sqrt{21}}\\\\
3\sqrt{21}(320-x^2)=8(480-x^2)\\\\
3\sqrt{21}\times320-3\sqrt{21}\;x^2\;=\;3840-8x^2\\\\
960\sqrt{21}-3840\;=\;(-8+3\sqrt{21})\;x^2\\\\$$
$$\\x^2=\frac{960\sqrt{21}-3840}{-8+3\sqrt{21}}\\\\
x=+\sqrt{\frac{960\sqrt{21}-3840}{-8+3\sqrt{21}}}\\\\$$
$${\sqrt{{\frac{\left({\mathtt{960}}{\mathtt{\,\times\,}}{\sqrt{{\mathtt{21}}}}{\mathtt{\,-\,}}{\mathtt{3\,840}}\right)}{\left({\mathtt{\,-\,}}{\mathtt{8}}{\mathtt{\,\small\textbf+\,}}{\mathtt{3}}{\mathtt{\,\times\,}}{\sqrt{{\mathtt{21}}}}\right)}}}} = {\mathtt{9.864\: \!242\: \!223\: \!858\: \!689}}$$
so
The forth side is approx 9.86metres long
Since our answers are the same I guess we interpreted the question the same after all