It's really easy to define the functions.

Function GetAreaOfARectangle(ByVal Edge1 As Integer, ByVal Edge2 As Integer) As Integer
    Return Edge1 * Edge2
End Function

---

Dim Area As Integer = GetAreaOfARectangle(5, 8)
Console.Writeline(Area) 'Output: 40

Function Age(ByVal YourAge As Integer) As String   
    Select Case YourAge
        Case Is < 18
            Return("You are younger than 18! You are teen!")
        Case 18 to 64
            Return("You are older than 18 but younger than 65! You are adult!")
        Case Is >= 65
            Return("You are older than 65! You are old!")                
    End Select
End Function

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Console.WriteLine(Age(48)) 'Output: You are older than 18 but younger than 65! You are adult!