C# if statement not working!!

Ok, guys this might be stupid but I just cannot figure out what on earth is wrong with this simple code:

public class NewBehaviourScript1 : MonoBehaviour {

	float speed = 10;

	// Use this for initialization
	void Start () {
	
	}
	
	// Update is called once per frame
	void Update () {

		transform.Translate(Vector3.left * Time.deltaTime * speed);
		Vector3 pos = Camera.main.WorldToViewportPoint(transform.position);

		if (pos.x > 0) {
			speed * 3;
		}
	
	}
}

I get this error:

Unexpected symbol 3', expecting [‘,
<operator>', or identifier’

If however I change the multiplication to an additon or anything else(even if a Debug.Log) the error goes away. Whats wrong???

You have to write
speed = speed * 3;
then it should work.