Accrued expenses are those expenses that have occurred, but have not been paid or recorded
To determine the amount of adjustment, you need to determine the amount of supplies that were used during the period. From the $1,200 worth of supplies at the beginning of the year, subtract the amount of supplies that are still left at the end of the year to get supplies used ($1,200 - $400 = $800 supplies used). Debit Supplies Expense and credit Supplies for this amount.
When you have a credit balance and a credit adjustment, you add the amounts together (20,000 + 2,000 = $22,000) for the Adjusted Trial Balance.
By accidently omitting an adjusting entry for the use of the one month of rent from the prepaid rent account, you would be omitting a debit to Rent Expense (which if omitted Net Income would be overstated and Expenses would be understated) and you would be omitting a credit to Prepaid Rent (which if omitted the Balance Sheet Assets would be overstated).
By accidentally omitting an adjusting entry for salaries incurred but not paid, you would be omitting a debit to Salaries Expense (which if omitted Net Income would be overstated and expenses would be understated) and you would be omitting a credit to Salaries Payable (which if omitted the Balance Sheet liabilities would be understated).