Memory-Defense/Assets/Scripts/BulletMovement.cs

using System;
using UnityEngine;

public class BulletMovement : MonoBehaviour
{
    public float moveSpeed = 10;
    public float lifeTime = 2;
    private float _currentTime = 0;

    private void OnEnable()
    {
        _currentTime = lifeTime;
    }

    // Update is called once per frame
    void Update()
    {
        transform.Translate(Vector2.up * (Time.deltaTime * moveSpeed));
        _currentTime -= Time.deltaTime;
        if (_currentTime <= 0)
        {
            gameObject.SetActive(false);
        }
    }

}