web form 2010 app handling errors

In a vb.net 2010 web form application,
1. I want to add error handling logic to an existing function since I added the following code:
   Dim dDaysEnrolled As Double  = CDbl(_dt.Compute("sum(" & _dt.Columns(5).ColumnName & ")", ""))
  Dim dAbsentTotal As Double = CDbl(_dtDetail.Compute("sum(" & _dtDetail.Columns(4).ColumnName & ")", ""))  
The values in the 2 statements listed above are obtained by making a call to the database and storing the values in 2 different collections. I am just adding this code to existing collections that have already been setup.
Thus can you tell show me error checking code I can add to the logic listed above?
2. If that is not possible, I want to put a try call block in the existing following function that cotnains the code I listed above:
#Region "Public Function AttInformation(ByVal AttLett ...