Camera rotate to face target ?

Unity Engine
1

I have free camera that move and rotate around in level. In this script if tb = true, I want my camera to rotate to target. I need this camera rotate to face character it self. Like this image.

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public Transform avatar1; // character
public Camera cam;
public bool tb;

void Start()
{
    tb = false;
}

void Update()
{

    if (tb == true)
    {
        StartCoroutine(bu1());
    }

}

IEnumerator bu1()
{
    cam.transform.position = Vector3.Lerp(cam.transform.position, new Vector3(avatar1.transform.position.x + 6, 10, avatar1.transform.position.z + 7), 0.1f);
  // rotate camera to face character smoothly
    yield return new WaitForSeconds(2);
    tb = false;

}
2 
// Get a vector pointing in the direction we need. TIP: for
// vectors, a - b = c, where c is a vector that points in the
// direction of the first vector(which is a) in relation to b.
Vector3 desiredForward = avatar1.position - camera.position;

// Make it unit length!
desiredForward.Normalize();

// Build a look rotation from the desired forward.
Quaternion desiredRotation = Quaternion.LookAt(desiredForward);

// Smoothly blend from the cameras current rotation, to the
// desired rotation...
float smoothing = 5f;
camera.rotation = Quaternion.Slerp(camera.rotation, desiredRotation, smoothing * Time.deltaTime);