If a firm in a perfectly competitive market faces a market price of $7 and it decides to increase its production from 4000 to 12 000 units, the firm’s marginal revenue:

In a perfectly competitive market, the price is determined by the market and individual firms are price takers, meaning they have no control over the price. The price that the market determines is also the firm's marginal revenue.

Marginal revenue is the additional revenue that a firm receives from selling one more unit of a good or service.

In this case, the firm is in a perfectly competitive market and the market price is $7. This means that the firm's marginal revenue is also$7.

If the firm decides to increase its production from 4000 to 12 000 units, the firm's marginal revenue remains $7. This is because in perfect competition, the marginal revenue is constant and equal to the market price, regardless of the quantity of output.

So, the firm’s marginal revenue remains $7 even if it increases its production from 4000 to 12 000 units.