You question is a bit unclear, but I take it you are having trouble getting the row to insert. If so, a couple things to look at:
1. This line needs to have a single quote insert before the parenthesis to complete the syntax:
+ txt10_March.Text +")", conn);
change to:
+ txt10_March.Text +"')", conn);
Note: this method of creating sql statements will leave you exposed to sql injection attacks. To avoid that, I would recommend that you use parameters instead.
2. Are you certain this is the correct connection string? Try using a dot instead and see if that works:
string ConnectionString = "Data Source=S\\SQLEXPRESS;Initial Catalog=Income_Tax;Integrated Security=true;";
change to:
string ConnectionString = "Data Source=.\\SQLEXPRESS;Initial Catalog=Income_Tax;Integrated Security=true;";