Is it a function?

What is the best way to answer questions with a x expression in the question. for example 3x + 10x=?

Which of the following is the result of expanding the series

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