Bob Martin, P.A., collected cash on account from a client for whom the business had provided delivery

Question:

Bob Martin, P.A., collected cash on account from a client for whom the business had provided delivery services one month earlier.
Requirements
1. Why didn’t the business record revenue when it collected the cash on account?
2. Write two accounting equations to show the effects of
a. receiving cash of $500 for service revenue earned.
b. receiving cash of $500 from a customer on account.