Adam earns $75 per week mowing lawns. This can be represented by the function f(x) = 75x. Each week he deposits this money into his bank account. At the beginning of January his bank account had a balance of $500 in it. Adam also makes a $45 withdrawal each week. The withdrawals from his account can be represented by the function f(x) = 500 - 45x. Write a function that can be used to represent his bank balance after n months, including his monthly deposit and withdrawal.