You get everything you need from factoring the last expression:

The left side is a difference of squares, and we get another difference of squares upon factoring. We end up with

Plug in everything you know and solve for
:

First, let's find all the factors of 81:
81/3=27
27/3=9
9/3=3
3/1=1
So the factors of 81 are 3,3,3,3, (the numbers by which we divided here).
This means we can write down 81 as 3*3*3*3 or

- which is the answer to the question! (three to the power of 4)
Well to find the value of the interior angle of a hexagon it would be out of a total of 720 degrees therefore just subtract the values of the known interior angle values and you should be able to get the value of the interior angle of X