get sum in special cell in vb.net

Well, you seem to know how to get the cell from a DataGridView... The problem is that a cell holds an Object. And you can't add Objects.
If you're sure cells (2, 3) and (3, 5) always exist and are always filled with a numeric type you could simply do the following:

Dim var As Decimal = CDec(dgvMyGrid.Item(2, 3).Value) + CDec(dgvMyGrid.Item(3, 5).Value)
txtMyTextBox.Text = var.ToString

If the values are Integers you could use CInt. It would be safe to check if the cell exist though...

If dgvMyGrid.Item(2, 3) IsNot Nothing AndAlso dgvMyGrid.Item(3, 5) IsNot Nothing Then
   ' Same code as above. Notice that if the value of the cell is Nothing CDec will return 0.
End If

Hope that helps.