The journal entries would be:
1.
Cash a/c..Dr$900
To service revenue $900
2.
Accounts payable a/c..Dr$8000
To cash $8000
3.
Supplies a/c..Dr$600
To accounts payable $600
4.
Cash a/c..Dr$2300
To accounts receivable $2300
5.
Accounts receivable a/c..Dr$5500
To service revenue $5500
6.
Office Rent ..Dr$1200
To cash $1200.
Whenever cash is paid;it gets reduced and hence credited and vice-versa.Similar with accounts receivable ie when cash is received from them,such balances reduce and are credited .Office rent paid is debited and hence increases expenses for the period etc.