Make make gameObject invisible until collision with bullet

Hey trying to make to a script for making a wall in my game be set to deactivating it at the start of the game. And upon detection of a bullet passing through it will destroy the bullet & will make the wall become active and appear before disappearing and becoming inactive after specific amount of time.

Code below is what I have so far. How would I go about making it work?

 public class AppearingWall : MonoBehaviour
{

// Use this for initialization
void Start()
{

    gameObject.SetActive(false);

}

public void OnTriggerEnter(Collider col)
{
    int countDown = 0;

    if (col.tag == "Bullet")
    {

        gameObject.SetActive(true);
        countDown++;

        if (countDown > 5)
        {
            gameObject.SetActive(false);
        }
    }

}

}

try something like this instead:

 public class AppearingWall : MonoBehaviour
 {
	 void Start()
	 {
		 renderer.enabled = false;
	 }
	 
	 public void OnTriggerEnter(Collider col)
	 {
		 int countDown = 0;
	 
		 if (col.tag == "Bullet")
		 {
			renderer.enabled = true;
			countDown++;
	 
			 if (countDown > 5)
			 {
				 renderer.enabled = false;
			 }
		 }
	 
	 
	 }
 
 }