To find equity in balance sheet, can we use total stockholder equity or there are some figure we need to plus?

Can we assume account receivable as credit sales?

Equity is what the owners have put into the business or what was retained in the business. So you would use Total Owners Equity unless your problem states that you have to find a part of it like net income or additional paid-in capital.

Accounts Receivables result when a ccustomer buys something from you on credit, so accounts receivables is only created from credit sales.