If your Format method is not shared, but only Public, you can access it only if you create an instance of your class.
For example, let's call 'I' the instance and 'MyC' your class

VB Code:
  1. Dim I as new MyC
  2.  
  3.  Dim s As String
  4.  
  5.         s = I.Format(txtamt.Text, "Standard")