Move an object from point A to B?

Hello, when I click the mouse, I want to move an object outwards from point A in the direction of point B at a set velocity. The game is 2D (along the ZX plane) with an over head camera, and I want point B to be wherever the mouse is clicked on the screen, how can that be done.

I have tried this;

var projectile : Rigidbody; 
var speed = 2; 
function Update() 
 if( Input.GetButtonDown( "Fire1" ) ) 

  var instantiatedProjectile : Rigidbody = Instantiate( 
   projectile, transform.position, transform.rotation ); 

  instantiatedProjectile.velocity = 


But using this just makes the object move in a weird direction (sometimes even at the camera) I have also tried to work in "Input.mousePosition" but I cannot get that to work. Thank you.

You need to translate from screen to world space somehow. You can do this by Camera.ScreenToWorldPoint and raycasting to hit a collider as in the fourth and fifth examples here