First you add the three static variables as instructed:
 private static double price;
 private static int discountRate;
 private static double tax;
Then you implement the sale() method:
public static double sale(){
 System.out.printf("Price before discount: %.2f\\", price);
 double sale = price * (1-(double)discountRate/100.0);
 System.out.printf("Price after discounting %d%% but before tax: %.2f\\", discountRate, sale);
 sale *= (1+tax/100.0);
 System.out.printf("Discounted price with %.1f%% tax added: %.2f\\", tax, sale);
 return sale;
}
I kind of took a stab at how to calculate the sale. Not sure if the tax has to be added before or after the discount. But I'm sure you can fix that if needed.