Returning values from functions in structures in Visual Basic

The way you have this setup those functions are creating a new, empty ComNum structure each time you call them (named either add or subt based on the function name). Unless you manually return them early it will just default to returning the function named structure.

    Function add(ByVal br As ComNum) As ComNum
        add.Re = br.Re + Re
        add.Im = br.Im + Im
    End Function

Is basically doing the equivalent of:

    Dim add As New ComNum
    add.Re = br.Re + Re
    add.Im = br.Im + Im
    Return add

Though like Lars pointed out I'm not sure why you'd want this to be a function vs a sub. Using it the way you have it setup now requires you to do something like this to get the add/subtract values because you need to capture the returned ComNum object.

    Dim a As New ComNum With {.Im = 1, .Re = 1}
    'Im = 6, Re = 6
    a = a.add(New ComNum With {.Im = 5, .Re = 5}) 

Doing something like this makes more sense to me.

Structure ComNum
    Dim Re As Double
    Dim Im As Double
    Sub add(ByVal br As ComNum)
        Re += br.Re
        Im += br.Im
    End Sub
    Sub subt(ByVal br As ComNum)
        Re -= br.Re
        Im -= br.Im
    End Sub
End Structure

Then you could just call it this way to update the struct without having to capture the returned values.

a.add(New ComNum With {.Im = 5, .Re = 5}) 

Edit: Knowing more now how the exercise is supposed to be performed I'd suggest something like this for the struct:

    Public Overrides Function ToString() As String
        Return String.Format("Re: {0}  Im: {1}", Re, Im)
    End Function

Then you could call the .ToString() method like this. Though, just a thought.

    Console.WriteLine("The second number added twice and subtracted once from the first number gives {0}", c.ToString())