double a = 1.0;
double b = 2.0;
double c = -4.0;

double res1 = 0;
double res2 = 0;

res1 = (-b + Math.sqrt(b * b - a * c * 4)) / a * 2;
res2 = (-b - Math.sqrt(b * b - a * c * 4)) / a * 2;

System.out.printf("res1=%.2f,res2=%.2f", res1, res2);