Ineed two know the work and the answer don't remember?

In the polynomial below, what number should replace the question mark to produce a difference of squares? x2 + ? x - 36

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

Asap! i'm not good math and i'm lost.