Plz ! combine as indicated by the signs. write variables alphabetically in your answer. (x-y/x+y) - (x+y/x-y)

Jada has a meal in a restaurant she adds up the prices listed on the menu for everything they ordered and gets a subtotal of $42.00. when the check comes, it says they also need to pay $3.99 in sales tax. what percentage of the subtotal is the sales tax