A store offers packing and mailing services to customers. The cost of shipping a box is a combination of a flat packing fee of $5 and an amount based on the weight in pounds of the box, $2.25 per pound. Which equation represents the shipping cost as a function of x, the weight in pounds?

f(x) = 2.25x + 5
f(x) = 5x + 2.25
f(x) = 2.25x − 5
f(x) = 5x − 2.25