Well that's not a polynomial then, since it has non-integer exponents. I guess it could *perhaps* be boiled down to a cubic, but I'm not sure if that is still valid if we have a complex number as an exponent.

Anyway, if you can't solve it, it should remain continuous and derivable regardless of what "u" is, so if you can't find an algebraïc way do to it, you can just use a Newton approximation and solve for x