Why is my Math with pq-formula wrong or in-precise

Question

I want to do math with the pq-formula but for some reason everything works except that the numbers are different then they should be.

The Formula that I want to represent is: x1,2 = -p/2 ± √(p^2/4 -q)

• If I do the Math with a calculator and not with my program I get other results (I tried with calculator and by hand)

Example with p=12 & q=-12:

what I get: x1 = 9.0 and x2= -21.0 what I want to get (does not have to be exact the same but close): x1 = 0.92820323027551 x2 = -12.928203230276

(teil means part; null means zero)

public class PQ {
  public static void main(String[] args) {
    if (args.length != 2) {
      System.out.println("ERROR: Bitte genau zwei Argumente angeben");
      return;
    }

    double p = Double.parseDouble(args[0]);
    double q = Double.parseDouble(args[1]);
    double teil1 = -(p / 2);
    double teil2 = (Math.sqrt(p * p) / 4 - q);
    double teil3 = (p * p / 4 - q);
    double Null1 = teil1 + teil2;
    double Null2 = teil1 - teil2;

    if (teil3 < 0) {
      System.out.println("Es gibt 0 Nullstellen.");
    }

    if (Null1 > Null2) {
      System.out.println(Null2);
      System.out.println(Null1);
      System.out.println("Es gibt 2 Nullstellen.");
    } else if (Null1 < Null2) {
      System.out.println(Null1);
      System.out.println(Null2);
      System.out.println("Es gibt 2 Nullstellen.");
      return;
    }

    if (Null1 == Null2) {
      System.out.println(Null1);
      System.out.println("Es gibt 1 Nullstelle.");
    }
  }
}

I tried changing the formula but there are always the same results