a
\(\begin{array}{l}
mg\,\sin \,\theta  + 2 = \mu \,mg\,\cos \theta \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,...\left( 1 \right)\\
10 - \,mg\,\sin  = \mu mg\cos \theta \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,...\left( 2 \right)\\
On\,adding:\,\,\left( 1 \right) + \left( 2 \right)\\
12 = 2\mu mg\frac{{\sqrt 3 }}{2}\,\,\,\,\,\,;\,\,\,\,\,\,\,\mu mg = \frac{{12}}{{\sqrt 3 }}\\
On\,\left( 1 \right) - \left( 2 \right):\\
8 = 2\,mg \times \frac{1}{2}\,\,\,\,\,;\,\,\,\,\,mg = 8\,\,\,;\,\,\mu  = \frac{{\sqrt 3 }}{2}
\end{array}\)