Final answer:
If a firm in a perfectly competitive industry tries to charge $21 when the equilibrium price is $15, it will not sell any output because consumers have many other options to buy the product at the lower market price.
Step-by-step explanation:
In a perfectly competitive industry, firms are price takers, meaning they must accept the equilibrium price set by the market's supply and demand. The equilibrium price is where the quantity demanded by consumers equals the quantity supplied by producers. As such, if a firm in this market attempts to charge $21 for their product when the equilibrium price is $15, it will face a situation where the demand for its product at that price point is zero. Consumers have perfect information and can obtain the product at the market price from countless other firms, so they will not purchase the product for more than the equilibrium price.
Therefore, if a firm charges $21 when the equilibrium price is $15, it will not sell any output. This happens because it faces a perfectly elastic demand curve, with plenty of substitutes available at the lower market price. Thus, the firm's profits will not increase, it will not sell more output than its competitors, and its revenue will not increase. In fact, its revenue is likely to decrease to zero unless it adjusts its price back to the equilibrium level.