Hi there!
We can use the conservation of momentum for a perfectly inelastic collision to solve.
[tex]m_1v_1 + m_2v_2 = v_f( m_1+ m_2)[/tex]
m1, v1 = mass and velocity of bullet (0.035 kg and ? m/s respectively)
m2, v2 = mass and velocity of block (5 kg and 0 m/s respectively)
vf = combined final velocity (8.6 m/s)
Plug in the givens and solve:
[tex]0.035 (v_1) + 5(0) = 8.6(0.035 + 5)\\
\\
0.035v_1 = 43.301\\
\\
v_1 = \frac{43.301}{0.035} = \boxed{1237.17 \frac{m}{s}}[/tex]