a
Energy lost over path \(\mathrm{PQ}\) is \(=\mu \mathrm{mg} \cos \theta \times 4\)

Energy lost over path \(\mathrm{QR}\) is \(=\mu \mathrm{mgx}\)

\(\mu \mathrm{mg} \mathrm{x}=\mu \mathrm{mg} \cos \theta \times 4\)

\(x=\cos \theta \times 4\)

\(\mathrm{x}=2 \sqrt{3}=3.45 \mathrm{m}\)

From \(Q\) to \(R\) energy loss is half of the total energy loss.

\(\mu \operatorname{mg} \mathrm{x}=\frac{1}{2} \times \mathrm{mgh} \Rightarrow \mu=0.29\)