I would recommend setting your double values when you declare them.
I would also write it so that you can enter a, b and c from the command line.
By doing that you could script it later and it would be more useful than having someone sit at a keyboard typing numbers.
If your only calculating one number, and the computer is waiting on user input, then I think your code is fine. If your planning to calculate a million numbers, you might notice a small speed increase if you combine line 16 with 17 and 18, line 17 with 19 and line 18 with 20.
Remove line 23, it's not needed, if you run from a prompt.
BTW, there seems to be a problem with your formula n=sqrt((b*b)-(4*a*c));