Do you expect three solutions, or just the real one? What about for x^2=-1, two or none? Is \sqrt[3]{7} enough, or is a decimal approximation required? If it isn't (and the student doesn't know how to go about it) one could argue that isn't much of an answer, but rather an existence claim, and that the methodology doesn't amount to much.
Personally I feel that you probably wouldn't give a question such as x^3=27 (neither would I), but if you did, marking it as wrong (as in no credit) after seeing the justification 1^3=1, 2^3=8, 3^3=27 would be too harsh. You can't penalize a student for giving out an easy question.
