Timeline of events: 1 June: Shenan mails a letter to Marcus offering to sell him his car for $5000 provided that he accepts by 15 June. 12 June: Marcus mails back his response via registered post. He accepts and encloses a $5000 cheque. 14 June: Laraib sees the car on the street and verbally offers Shenan $6000 cash for the car. 14 June: Shenan then writes another letter to Marcus. In the letter he cancels his original offer sent 1 June. He then rings Laraib and accepts her offer. 16 June: Shenan receives Marcus's registered post envelope containing $5000 cheque and acceptance letter. 17 June: When Marcus receives Shenan's second letter, he rings Shenan and says he will be seeking advice from his solicitor, Gibson.

Who's contract will be valid Marcus or Laraib? how will the solicitor Gibson advise Marcus in what to do