How to rotate object based on mouse position

I'm making a game and in it you are suppose to shoot enemies . So I need the arm to rotate based on the mouse position. So I thought I could calculate the angle by finding the distance from the mouse to the center of the object (with vector2.distance). Then I thought I could find the position from follow to center . After that I divided follow to center by mouse to center(to simulate cos). Then I did acos to find the angle,and then I set the angle to the Z of the rotation.

 using UnityEngine;
 using System.Collections;
 
 public class Test : MonoBehaviour
 {
     public Transform center;
     public Transform follow;
     void Start() {
 
 
         
         
 
     }
 
     void Update () {
         //mouse position
     
         Vector3 mousepos = Input.mousePosition;
 
         //2d positions
         Vector2 centpos = new Vector2 (center.position.x,center.position.y);
         Vector2 mousepos2d = new Vector2 (mousepos.x,mousepos.y);
         Vector2 folpos = new Vector2 (follow.position.x,follow.position.y);
 
         //distances
         float hyp = Vector2.Distance (mousepos2d,centpos);
         float adj = Vector2.Distance ( folpos,centpos);
 
         //angle calculations
         float c = adj / hyp;
         float angle = Mathf.Acos (c);
 
         //setting the angle as rotation
         Vector3 rotation = new Vector3 (0f,0f,angle);
         gameObject.transform.Rotate (rotation);
     }
 }

But my end result is just my gameobject rotating continuously.