Things seem to be going wrong once you've substituted. Why are you squaring the height (6*h)?

Let me show you how to do it:

Where brackets signify substitution.

First, I'll multiply both sides by and divide both sides by :

Next, I'll distribute the squared out:

The four from what I just did cancels with the . Also, I will combine the and the .

Take the third root of both sides, and you're done!