I can't fix this error!

I’m getting an error: paddleScript.cs(42,31): error CS0200: Property or indexer `UnityEngine.GameObject.rigidbody’ cannot be assigned to (it is read only)

Here is my script:

using UnityEngine;

using System.Collections;

public class paddleScript : MonoBehaviour {
	
	 float paddleSpeed = 10f;
	 public GameObject ballPrefab;
	 private Vector3 ballPosition;
     private Quaternion ballRotation;
	
	
	 GameObject attachedBall = null;
	
	// Use this for initialization
	void Start () {
		
		
	SpawnBall();	
	
	}
	
	void SpawnBall() {
		 Vector3 ballPosition;
		 attachedBall = ballPrefab;
		 	
		 
		//Spawns the ball, duh!
		
		ballPosition = transform.position + new Vector3(0, 1f, 0);
		Quaternion ballRotation = Quaternion.identity;	
	   
		
		attachedBall = (GameObject)Instantiate( ballPrefab, ballPosition, ballRotation );
	}	
	
	// Update is called once per frame
	void Update () {
	
		transform.Translate ( paddleSpeed * Time.deltaTime * Input.GetAxis ( "Horizontal" ), 0, 0 );
		
		if( attachedBall ) {
		 attachedBall.rigidbody = transform.position + new Vector3(0, .75f, 0);	
		}	
		if( Input.GetButtonDown( "Jump" ) ){
		   //Fire the ball!
		   if( attachedBall ) {
		   attachedBall.rigidbody.isKinematic = false;		
		   attachedBall.rigidbody.AddForce(0, 300f, 0);
		   attachedBall = null;		
			
		}
    }
}		
	
	
	void  OnCollisionEnter( Collision col ) {
	    foreach (ContactPoint contact in col.contacts) {
		if( contact.thisCollider == collider ) {
			 	float english = contact.point.x - transform.position.x;
				
				contact.otherCollider.rigidbody.AddForce( 300f * english, 0, 0);
			}
			
		}
	}	
}

You are trying to put a Vector3 into a rigidBody. Stop that. If you want to change the position, put that value into transform position.