Convert double to string
a = 0.000006;
b = 6;
c = a/b;
textbox.Text = c.ToString("0.000000");
As you requested:
textbox.Text = c.ToString("0.######");
This will only display out to the 6th decimal place if there are 6 decimals to display.
Try c.ToString("F6");
(For a full explanation of numeric formatting, see MSDN)