Extension Methods on Mathf not working

I'm trying to make some extension methods to Mathf that rounds, floors or ceils a value by a specific base (which base = 1 reduces to normal round/floor/ceil). Except that the code's not working, and the methods are at my extension methods class rather than Mathf struct:

 using UnityEngine;
 using System.Collections;
 using System;
 
 public static class ExtraMethods {
     
     public static double Round2(this Mathf math, double num, double round)
     {
         return System.Math.Round (num / round) * round;
     }
     
     public static double Floor2(this Mathf math, double num, double round)
     {
         return num - num % round;
     }
 
     public static double Ceil2(this Mathf math, double num, double round)
     {
         return num - (num % round) + round;
     }
     
 }