Whats the function value f(-3) if f(x)=2x-8 ?

Can some one me with this math problem?

Suppose a store sells an average of 80 cases of soda per day. assume that each case contains 24 cans of soda and that the store is open 10 hours per day. find the average number of soda that the store sells per hour.

Leslie started last week with $1200 in her checking account. during the week, she wrote the checks below.