Cc = 1 - (1 / (2 * n + 1)) * (YP / (YP + K * ((3 * n + 1) * q / (n * WorksheetFunction.Pi * (ID / 2) ^ 3)) ^ n))

var newInstruction = string.Empty;
var newCodeLines = string.IsNullOrEmpty(newInstruction)
    ? string.Empty
    : originalCodeLines.Replace(originalInstruction, newInstruction);

var newCodeLines = originalCodeLines.Replace(originalInstruction, string.Empty);

foo = Range("A1") ' should be implicit reference to ActiveSheet

foo = Sheets("Sheet1")

Private WithEvents app As Application

Private Sub app_NewWorkbook(ByVal Wb As Workbook)
    ' he he he...
End Sub