Why not just use a Select Case like this :
Dim intToCalculate As Integer = 0
Integer.TryParse(txtBasic.Text, intToCalculate)
Select Case cboSelect.SelectedIndex
Case 0
txtResult.Text = intToCalculate * 2
Case 1
txtResult.Text = (intToCalculate * 2) * 2
Case 2
txtResult.Text = (intToCalculate * 10) / 2
End Select
If you need to use it in more than one place then create a function like this:
Enum FormulaType
Formula1
Formula2
Formula3
End Enum
Private Function FormulaResult(ByVal intToCalculate As Integer, ByVal yourFormulaType As FormulaType) As Integer
Dim intReturn As Integer = 0
Select Case yourFormulaType
Case FormulaType.Formula1
intReturn = intToCalculate * 2
Case FormulaType.Formula2
intReturn = (intToCalculate * 2) * 2
Case FormulaType.Formula3
intReturn = (intToCalculate * 10) / 2
End Select
Return intReturn
End Function
Hope this helps.
Happy Coding :)