Which equation is a function of x? x = 5 x = y squared + 9 x squared = y x squared = y squared + 16

What is the equation of the graph below? y=sec(x)-4

Pls! does anybody know a shortcut for answering these types of questions in the future?

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! )