As I wrote in my Comment :
Dim mySum as double = cdbl(TextBox1.Text) + cdbl(TextBox2.Text) + cdbl(TextBox3.Text) +cdbl(TextBox5.Text)
table.Rows.Add(mySum.toString)
But note :
Each Textbox-Value MUST be able to be converted. If it is possible that there is a non-decimal value in it you should handle it.
the Result mySum has no Formatting - but toString gives you the ability to do this - see MSDN-Help for this ...