Final answer:
The equation that describes a firm's net cash flow from operating activities is Net cash flow - Operating income - Depreciation.
Step-by-step explanation:
The equation that describes a firm's net cash flow from operating activities is Net cash flow - Operating income - Depreciation. This equation takes into account the cash flow generated from the firm's operating activities, such as sales revenue and expenses, and subtracts the operating income (profit) and depreciation of assets.