Use
Math.Round[
^]
"float m1 = Convert.ToInt32(TextBox11.Text);
float m2 = Convert.ToInt32(TextBox12.Text);
float m3 = (m1 / m2) * 100;
then where i use math.round....??"
Why are you converting to floats, using ToInt32?
Use this instead:
float m1 = Convert.ToSingle(TextBox11.Text);
float m2 = Convert.ToSingle(TextBox12.Text);
float m3 = (float) Math.Round((m1 / m2) * 100, 3);
Or better use TryParse to convert them, as you could then report a user error if he enters "Hello" instead of "123.45".
In addition, you should check your inputs: if the user puts "0" in TextBox12, then the code will throw a Divide by Zero exception.
BTW:
1) Deleting your question when it has been replied to is very rude, and can make people unwilling to help you again.
2) Please don't use the default names that VS supplies: change them to something more meaningful. You may understand what is supposed to be in "TextBox7" today, but next week it may be a blur. Calling it "inputUserName" or whatever makes your code more readable, and easier to maintain.
3) I really meant it about Parameters.Add being depreciated in favour of Parameters.AddWithValue.