\([\) after bullet gets embedded till the system comes momentarily at rest \(]\)

\(( M + m ) g h =\frac{1}{2}( M + m ) v _{1}^{2}\)

\(\left[ v _{1}\right.\) is velocity after collision]

\(\therefore v _{1}=\sqrt{2 gh }\)

Applying momentum conservation, (just before and just after collision)

\(mv =( M + m ) v _{1}\)

\(v =\left(\frac{ M + m }{ m }\right) v _{1}=\frac{6}{10 \times 10^{-3}} \times \sqrt{2 \times 9.8 \times 9.8 \times 10^{-2}}\)

\(\approx 831.55\, m / s\)