Use the given graph of f to estimate the intervals on which the derivative f ' is increasing or decreasing. (Enter your answers using interval notation.)

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! )