How to determine the price a monopolist will set when the demand curve the firm faces isn't known?
The monopolist is allowed to set the price. However, because his product has a perfect substitute, he can sell 0 products at prices $p&gt;2$. He also has no incentive seting the price $p&lt;2$, because the consumers will buy any number of products he wishes to sell at $p = 2$. This means that $p(q) = 2$. Its graph in the $(q,p)$ coordinate system would be a horizontal line at $p = 2$. Because of this, $p'(q) = 0$. Correcting for this you should be able to easily finish the exercise.