An analyst uses a mathematical model to find a firm's profit-maximizing level of production. The calculation correctly solves for the point where marginal revenue equals marginal cost, but the result is a negative quantity of output (e.g., -50 units), which is physically impossible. Explain how sketching a diagram of the firm's relevant cost and revenue curves would help to interpret this mathematical result. What specific feature of the diagram would clarify the firm's best course of action?