Return to Revisions

After taking some great advice on this question, I'm hoping for some more feedback on my quadratic expression calculator.

This is my code as stands:

public class Quadratic_Equations {
 private final static Scanner sc = new Scanner(System.in);
 public static void main(String[] args) {
 while (true) {
 System.out.println("Please enter a, b and c or enter \"exit\" at any time to exit");
 double a = promptUserUntilValidInput("Please enter a:");
 double b = promptUserUntilValidInput("Please enter b:");
 double c = promptUserUntilValidInput("Please enter c:");
 System.out.println(formatOutput(calculateQuadraticFormula(a, b, c)));
 System.out.println();
 }
 }
 private static String formatOutput(Set<Double> resultsFromEquation) {
 if (resultsFromEquation == null)
 return "These numbers do not compute - they produce an illegal result.";
 else {
 return resultsFromEquation.toString();
 }
 }
 private static double promptUserUntilValidInput(String prompt){
 if (prompt.equals("exit")) {
 sc.close();
 System.exit(0);
 }
 boolean valid = false;
 Double value = new Double(0.0);
 while(!valid){
 System.out.println(prompt);
 value = tryParse(sc.nextLine());
 if (value != null)
 valid = true;
 }
 return value;
 }
 private static Double tryParse(String text){
 try{
 return Double.parseDouble(text);
 }
 catch (NumberFormatException e){
 return null;
 }
 }
 public static Set<Double> calculateQuadraticFormula(double a, double b, double c) {
 Set<Double> results = new HashSet<Double>();
 double temp = (b * b) - (4 * a * c);
 if (temp < 0)
 return null;
 if( temp == 0 ) {
 results.add(-b / 2 * a);} 
 else if( temp > 0) {
 results.add( (-b + Math.sqrt(temp) ) / (2 * a) );
 results.add( (-b - Math.sqrt(temp) ) / (2 * a) );
 }
 return results;
 }
}

Before I go on, there are a few things I should point out.

I'm aware this could be better if the quadratic stuff was in a class of its own - however, for the purposes of this simple exercise I'm happy skipping that.

My main issues are:

1. static Scanner. Some people on previous question suggested making this a private variable, but then it needs to get passed to everything. Is this worth it?

2. Does it look okay to close the scanner (and exit the program) where I do?

3. Do other methods look single-responsibility-ish enough?