Calculating Bullet Speed Before Impact for Vibration Problem

[confusedaboutphysics]

A 20.0 g bullet strikes a 0.600 kg block attached to a fixed horizontal spring whose spring constant is 6.74 103 N/m and sets it into vibration with an amplitude of 21.2 cm. What was the speed of the bullet before impact if the two objects move together after impact?

I've been stuck on this question for days! help please!

 

[Päällikkö]

For the block+bullet -system to oscillate with the given amplitude, what must the (initial kinetic) energy of the system be?

During impact energy is not conserved, though.