How to move a game object using the mouse

Ideally, id like it so that when I hold down A, the cube moves to the left, and when I hold down D, the cube moves to the right. Ive tried for about an hour now, and this is what I have:

 function Start () {
     var positionx Input.mousePosition.x;
 
 }
 
 function Update () {
     gameObject.transform.position.x(positionx);
 
 }

I get an error telling me that I need a semi colon at the end of var positionx Input.mousePosition.x;, although i already have one... + it just straught up doesnt work