Final answer:
The profit-maximizing output level for the firm is 9 units.
Step-by-step explanation:
The profit-maximizing output level for the firm can be determined by setting marginal revenue equal to marginal cost. In a perfectly competitive market, where price is equal to marginal revenue, this equation can be simplified to: P = MR = MC. Using the provided information, the marginal cost equation is MC = q² + 9. To find the profit-maximizing output level, we need to find the quantity (q) where marginal cost is equal to the market price (P). Substituting the given market price of $90 into the marginal cost equation, we get:
90 = q² + 9
Subtract 9 from both sides:
81 = q²
Take the square root of both sides:
q = ±9
Since quantity cannot be negative in this context, the profit-maximizing output level is q = 9 units.