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You tried to take the square root of a negative number. NaN stands for "not a number", which technically isn't true. It's just not a real number, but there are imaginary roots.

:O


public class Ø
{
public Ø()
{
System.out.println("No!");
}
}

I think you might run into a problem by naming a variable quadratic and the class quadratic... not positive on this. As a general rule, always name classes starting with upper case letters, and...

... they are the coefficients to the quadratic equation. In other words, you (or the user) gets to choose them. I don't see much use in this program, it can only solve one quadratic equation :P
...

You're java syntax is a bit off :P Also, it's not possible for a method to return 2 values, so I removed the helper method. Don't know if you have to handle imaginary roots, I left that out. I would...

What have you done so far?