In some cities you can rent a car for $15 per day and $0.40 per mile. (a) find the cost of renting the car for one day and driving 400 miles. (b) express the total rental expense as a function of the miles driven (x) when the car is rented for one day.

So the cost for renting the car for one day and driving it for 400 miles would be around 15 + 400 (0.4) = 160 + 15 = 175$

To write an expression for a day you can obviously just make up one by putting in x as the number of miles. So to calculate the cost for x miles you can write x * (0.4) and more over at the end we have to add the 15$ so the expression equals to 15 + 0.4 (x)