Please help and explain how you got the answer, please and thanks

What is the value of x in the diagram below?

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

Solve for x: 5/3 x = 20/21 it's fraction. remember to simplify your fraction