Physics help Please!?

The same amount of kinetic energy, calculated as (1/2)mv^2, will be applied to both the gun and the bullet. You need to use the mass and the velocity of the bullet to figure out how much kinetic energy is generated, then plug that amount into the same KE=(1/2)mv^2 formula to figure out how fast the gun will be traveling. Once you know the mass of the gun and the amount of kinetic energy applied to it, you can solve for the velocity.