Your formula needs to be in quotation marks. Without them, Perl tries to interpret the expression instead of the MathObjects parser.
Note, however, that you will not be able to enter the equation in the LimitedPolynomial context, since it is not a simplified polynomial. You need to provide the answer in the required format (so that WeBWorK can give a correct answer when requested).
Finally, your Context("Numeric") and Context() variables settings will have no effect, since no MathObjects are created between there and the following Context("LimitedPolynomail") call.