Harry saw Joe in his classic Buick at a local drive-in restaurant and talked about wanting to buy it. Joe offered to sell it to him for $1500. Harry said he needed some time to think about it and that he would send him a letter. The next day, Joe changed his mind and mailed Harry a letter of revocation. The day after that, before Harry had received the letter of revocation, he sent a letter of acceptance to Joe. The next day the two letters arrived at their various destinations, Joe receiving Harry's letter of acceptance just after Harry received Joe's letter of revocation.

a) Explain the legal liabilities of the parties.

b) Explain whether there is a valid contract between the parties based on the principles of contract law.