Round two decimals
-2x^4+9=3x+5
since x has an unspecified value, there is no way to round it. So, I assume you want to solve the equation, and round the answer to two decimals.
2x^4+3x-4 = 0
There are only two real roots,
x = -1.43 or 0.90
The graph is at
http://www.wolframalpha.com/input/?i=2x^4%2B3x-4
Or, you can use your favorite calculator or numeric method to solve for x.