What is the purpose of a regression line?

Me because some of question i don’t understand

I'm at a bookstore with $\$35$ in my pocket and a $20\%$-off coupon. what is the cost of the most expensive book i can buy with my money and the coupon? (assume that there are books of every possible price in the bookstore. it's a large bookstore! )

Can anyone explain to me how to do this? anything will .