Help with script for Bullet velocity

Hello! I'm trying to get a simple script where I can input the initial velocity of a bullet (in m/s) that decreases over time. Here's what i've got(doesen't work very well):

 sing UnityEngine;
 using System.Collections;
 
 public class BulletMove : MonoBehaviour {
 
     public float speed = 1f;
     public float drag = 1f;
 
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
         float velocity = ((speed * Time.deltaTime) -- (drag * Time.deltaTime));
         transform.Translate (0, 0, velocity);
         if (velocity -= 0) {
             transform.Translate (0,0,0);
         }
     }
 }
 

If anyone knows of a more simpler/easier way of doing this please do give some input! :D Thank you!