Sammy is classifying quadrilateral EFGH. How can she use coordinate geometry to determine whether EFGH is a parallelogram? E(0, 4), F(0, 7), G(1, 5), H(1, 2)

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! )