Firm X is a monopolist which is currently maximizing profit by selling its output at a price of $64/unit.

If elasticity of demand is -9, use the mark-up formula to determine what Firm X's marginal cost must be.

Hello, the answer is 56.89.

However, I'm not sure how 56.89 was is calculated. I need the work to get 56.89 so I can understand like problems, thank you !