I can confirm that this question has appeared in the ATPL Gen Nav exam and that, as far as I know, it is still in the Common Question Bank and has every possibility of turning up again.
Gonville and Jockey have the right approach. The geoid shape of the Earth can be approximated closely to an ellipsoid. The lengths of the Polar Diameter and the equatorial Diameter are in the ratio 296/297. Therefore if you know one, you can work out the other. The question gives you all the information; it is not really your navigation knowledge which is being tested, but your ability to re-arrange simple algebraic formulae. This is quite usual in Gen Nav, which is more than a simple test of knowledge - it is a test of numeracy as well.
Polar semi-diameter = 6378.4 x 296/297
Alternatively, you should have been taught that the Polar diameter is 43 km shorter that the Equatorial. In which case, the semi-diameter will be shorter by half that figure. Half of 43 is 21.5 km.
6378.4 - 21.5 = 6356.9
Jinkster, you're going to have to accept that there is an awful lot of simple calculation in Gen Nav - you can't possibly learn all the answers to all the questions by heart. None of it is above GCSE level - it's simple re-arrangement of formulae, and nothing more complicated than sine, cos and tangent - but you've got to be proficient, quick, and accurate.