What is the initial velocity of the bullet and the time of flight of the bullet?
You fire the bullet VERTICALLY AGAINST GRAVITY. (You must disregard the effect of air friction which will slow the bullet down unless you are given a coefficient of friction as part of the problem). At 1000m the bullet STOPS. It then falls back to earth under the pull of gravity. Applying the acceleration attributable to gravity, (it used to be 32 feet per second per second when I was at school !), it will be so many metres per second per second , you should be able to calculate it’s speed after 1000m. This should then be equal to the muzzle velocity which propelled it to 1000m in the first place. This is an elementary solution but may suffice.