You can calculate the free cash flow using more than one way. The most common is the following:
Free cash Flow =
Earnings Before Interest and tax
Add Back Depreciation
Add Back Amortization
Less Movement in Accounts Receivable*
Less or Add Movement in Accounts Payable*
Less or Add Movement in Inventory*
Less Amounts spent to buy Property Plant and Equipment
* Note: The movement refers to current assets and current liabilities.
If your accounts receivable were $200 and your have 100$ this year, the next change is 200-100 or 100 (calculate as opening less closing). For the current liabilities, you take the closing less the opening.
The same thing applies (opening less closing) applies for all relevant current assets (including inventory). For current liabilities is the other way around. So basically, its closing less opening.