please help with this

Suppose a profit-maximizing monopolist is producing 900 units of output and is charging a price of $55.00 per unit. If the elasticity of demand for the product is - 3.00, find the marginal cost of the last unit produced. The marginal cost of the last unit produce is $ 36.67 . (Enter your response rounded to two decimal places.) What is the firm's Lerner Index? The firm's Lerner Index is . (Enter your response rounded to two decimal places.)