Trying to move an object between two points

Can you please help me arrange this correctly…

#pragma strict
public var speed = 5f;

function Start () {
	
}

function Update () {
		transform.position.x = (Mathf.PingPong (Time.time * speed, 5.1, 0.9));
	
}

Try this as a starting point:

transform.position = new Vector3(.9 + Mathf.PingPong(Time.time * speed, 4.2), transform.position.y, transform.position.z);

Now the object goes between 0 and 5.1 Not 0.9 and 5.1