Devin is having trouble with his assignment. His shown work as follows: -3 5 5 3= -3 5 x 5 3= - 15 15 = -1 however ,this answer does not match the answer that his teacher gives him. What is Devin mistake? Find the correct answer and show your work.

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! )