In your question you refer to $$23000^2$$ and in the title you refer to 23000 mm.
Anyway, 23000 mm = 2300cm and the formula for the volume of a cone is given by
$$V = \pi r^2 \frac{h}{3}$$
Depending on your question, either
$$2300 = \pi (\frac{40}{2})^2 \frac{h}{3}$$
or
$$2300^2 = \pi (\frac{40}{2})^2 \frac{h}{3}$$
which can be rewritten to
either
$$h = \frac{3*2300}{\pi (\frac{40}{2})^2}$$
or
$$h = \frac{3*2300^2}{\pi (\frac{40}{2})^2}$$
depending on you original question.
I think you can do the calculation yourself...
Reinout
