In the POD documentation
the issues you're dealing with are explained in some detail. In general, parserImplicitEquation.pl is not as reliable as we would like, and does not work well when all of the quantities in the equation are always positive. In this particular case, there is one preferred form of the answer, so I would recommend that you ask the question in the form
___________ = 1
where ____________ represents the answer blank. Giving them the right side of the equation would allow you to use a MathObject Formula for the answer instead of a MathObject ImplicitEquation.