Office supplies has a balance of $2400. An inventory at Dec 31 shows $1700 of supplies on hand. How do I adjust this in order to close for the financial statements?

First you will have to calculate the difference between what your account balance is and your actual balance.

Your journal entry will be:
Debit Office Supplies Expense for the amount above
Credit Office Supplies for the amount above

This journal entry will bring your account balance into agreement with your actual inventory balance.