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Looking good. Now divide through by x and rearrange to get x by itself.

Two issues with this. First, the second half of that first equation should be "- 1/2 g t^2", not "* 1/2 g t ^2." This appears to be a simple typo, based on subsequent equations, so no harm done. ...

I get something a bit different:

0 = \tan \phi - \tan \theta -x(\frac g 2 V_0^2\cos^2 \phi)

It seems you have the definitions of phi and theta reversed. Also, you didn't specify this but I...