I'm studying accounting equation now and I'm answering an exercise, identifying the effects of the following transactions on the assets, liabilities...

I'm studying accounting equation now and I'm answering an exercise, identifying the effects of the following transactions on the assets, liabilities and capital.

1. Billed a customer for service rendered

asset (+) = Liabilities (No Effect) + Capital (+)

2. Received payment from the customer in #1

asset (+/-) = Liabilities (No Effect) + Capital (No Effect)

I'm wondering why in #2 asset become +/-?