Because it's integer arithmetic - and it doesn't know anything about fractional parts!
Change all your
and it'lkl do what you expected:
double a = Convert.ToDouble(Textbox1.Text);
double b = 168;
Textbox3.text = (a / b).ToString();
BTW: The standard way to do any type to a string is to use the ToString method for the class rather than Convert.ToString - every type implements it, and your classes can override it to produce a meaningful message.
And rounding up is easy: Math.Ceiling will do it:
Will give you 8