Answer:
The initial speed of the bullet is
.
Step-by-step explanation:
The collision between bullet and block is inelastic and let suppose that motion occurs on a horizontal surface, so that changes in gravitational potential energy can be neglected. Initially, the intial speed of the bullet-block system can be determined with the help of the Work-Energy Theorem and the Principle of Energy Conservation:


The initial speed of the bullet-block system is:

Now, the initial speed of the bullet is determined by applying the Principle of Momentum Conservation:

