Please help! accounting question!?

A company reported annual sales revenue of $645,000. During the year accounts receivable increased from a $14,000 beginning balance to a $19,000 ending balance. Accounts payable decreased from a $13,000 beginning balance to a $6,000 ending balance. How much is cash received from customers for the year? I think it is 640,000 but i'm not sure why. please explain!