Factor x^4 -15x^2 +54

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! )

Which expression represents the total perimeter of her sandwich, and if x = 1.2, what is the approximate length of the crust?

Answer asap! urgent! will give brainliest!