Car rental company charges $30 a day and 12 cents a mile for its cars. (a) write a formula for the dollar cost, c, of renting a car as a function of the number of days, d, and the number of miles driven, m.

To write an expression for a day you can obviously just make up one by putting in m as the number of miles. So to calculate the cost for m miles you can write m * (0.12) and more over at the end we have to add the 30$ so the expression equals to 30 + m (0.12).

Now for more days we can simply multiply it by d. So d * (30 + m (0.12)) for some days d.

FInally, c = d * (30 + m (0.12)).