Perfect competition: Finding short run equilibrium price?
These kinds of questions depend heavily on the market structure that you assume. Most importantly, when firms have (some degree of) market power, they usually take the demand function as given and try to solve something of the kind

$$ \max_p D(p)\cdot (p-c(D(p)))$$

However here, when there is no market power, firms take the price as given and solve for the optimal quantity of production $q$:

$$ \max_q q\cdot (p-c(Q))$$

Then, we get the supply of any single firm, given the price $S(p)$ from solving this problem. To clear the perfectly competitive market, we remember that there are $30$ firms and solve:

$$p: D(p) - 30\cdot S(p) = 0$$