[DotNetDevelopment] calling method on a object

[DotnetBeginner]

Created a method and i would like to now how to call this change()
method on ob.Could any one help me on this
class Test
{
  //summation
    public int a, b;
     public Test(int i, int j)
     {
         a = i;
         b = j;
     }
    public void change(int c)
    {
        c= a+b;

    }
}
class Average
{
    public static void Main()
    {
        Test  ob = new Test(12,13);
        ob.change(-----);
        Console.WriteLine(----);
    }
}