\"\"

\

Draw a diagram for the situation:

\

\"\"

\

Apply the Cosine Law: \"\".

\

\"\".

\

The solutions of quadratic equation \"\" are \"\".

\

Here, \"\", \"\" and \"\".

\

\"\"

\

\"\"

\

\"\"

\

\"\"

\

\"\"

\

\"\".

\

Since \"\" is negative, does not consider.

\

\"\".

\

\"\"

\

\"\".