What is the solution to the system of equations

The lines shown below are parallel. if the green line has a slope of -1, what is the slope of the red line?

Can someone me out here number 4 plz

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