You are trying to estimate the fixed costs and variable costs for a firm by looking at its past financial statements. In order to estimate these costs, you must run a regression with:

Fixed assets as Y-variable, and costs as X-variable

Costs/ Sales ratio as Y-variable, and sales as X-variable

Costs as Y-variable, and fixed assets as X-variable

Costs as Y-variable, and sales as X-variable

Sales as Y-variable, and costs as X-variable