validate a procedure...?

Option Explicit
Dim value1 As Double
Dim value2 As Double
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Sub Midpoint(ByRef One As Double, ByRef Two As Double, ByRef Three As Double, ByRef Four As Double)
  value1 = Val(One + Two) / 2
  value2 = Val(Three + Four) / 2
  Answer = value1 & " , " & value2
End Sub
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Sub Quadratic(ByRef value1 As Double, ByRef value2 As Double, ByRef One As Double, ByRef Two As Double, ByRef Three As Double)
  value1 = (One * -1) + (Sqr((Two) ^ 2) - 4 * One * Three) / (2 * One)
  value2 = (One * -1) - (Sqr((Two) ^ 2) - 4 * One * Three) / (2 * One)
  Answer = value1 & " , " & value2
End Sub
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Sub SurfaceCylinder(ByRef One As Double, ByRef Two As Double)
  If One <> "" And IsNumeric(One) Then
  If Two <> "" And IsNumeric(Two) Then
    Answer = 2 * Pi * One ^ (2 + ((2 * Pi * One) * Two))
    Exit Sub
  Else
    MsgBox ("Please do not leave any fields blank."), vbInformation + vbOKOnly, "Missing Data"
  End If
  End If
End Sub
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Private Sub Solve_Click()
Select Case Action.Caption
  Case Is = "Surface Area of a Cylinder"
    Call SurfaceCylinder(One, Two)
  Case Is = "Midpoint Formula"
    Call Midpoint(One, Two, Three, Four)
  Case Is = "Quadratic Formula (-b +... , -b -...)"
    Call Quadratic(value1, value2, One, Two, Three)
  Case Else
    MsgBox ("Please select a function to perform from the menu"), vbOKOnly, "No Function Chosen"
End Select
End Sub