Adds to -1 multiplies to -12

Just tell me how to set up the equation.

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

1. find the area of the unshaded square 2. find the area of the large square 3. what is the area of the frame represented by the shaded region show