You're well on your way to the solution.

You have too many constants in your derivative of the cost function. There should only be 1 2880 and 1 1440. Otherwise it seems right.

Set C' = 0 and solve for x. The 'c' cancels out (you could also have set it equal to 1 at the start to make life easier - your answer doesn't depend on the actual cost of the wood, just the relative cost).