-- Momentum is conserved.
-- It doesn't magically appear.
-- It doesn't evaporate and disappear.
-- If new momentum shows up, it came from somewhere.
-- If momentum is missing, it went somewhere.
-- If no momentum came or went, then the amount of it is constant.
Momentum = (mass) x (speed)
Momentum AFTER = (mass of bullet + block) x (speed)
= (0.0758kg + 4.24kg) x (6.17 m/s)
= (4.3158 kg) x (6.17 m/s)
= 26.6285 kg-m/s (rounded)
Momentum BEFORE = the same amount, but the block isn't moving.
(mass of bullet) x (speed of bullet) = 26.6285 kg-m/s
(0.0758 kg) x (speed of bullet) = 26.6285 kg-m/s
Speed of bullet = (26.6285 kg-m/s) / (0.0758 kg)
= 351.3 m/s .