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)).
