Push back an enemy on trigger

I have two gameobjects, one of them is my "hero" an the other one the enemy with a NavMesh, I've managed to make the health system without problems but know when the sword collides with the enemy it seems that the rigidbody is not working or something because I've tried a couple of solutions that I've found on Internet but nothing seem to work. I want the enemy to be pushed once the sword collides with it. I've tried this:

 public void takeDamage() { 
   // revert rigidbody velocity:
   rigidbody.velocity *= -1;
 }

Another code

 float distance = 25.0f;
                 if (player.position.x < enemyPosition.position.x) {
                 Vector3 new_position = new Vector3 (enemyPosition.position.x + distance, enemyPosition.position.y, enemyPosition.position.z);
                 rb.position = new_position;
                 }

Thanks