Mine has got a page 449
ok, we've got HDG(T) - the true heading. HDG(M) is the magnetic heading, which is the true heading corrected for magnetic variation. Now, HDG(C), which is what you're trying to work out, is the corrected heading to steer - the heading that compensates for the compass error when the electrical systems in the aeroplane are on. Your aeroplane will have a card in it that will have the errors at the four or eight main points on the compass (have a look at page 38).
Now, my take on the question is that heading north you have +3 degrees of compass error (for N(360) steer 003), and heading east you have -3 degrees (for E(090) steer 087). You don't know exactly what the error is for a magnetic heading of 020, so you assume the error is linear and interpolate between the two points you do have (360 and 090) to get an error of +2 degrees, hence the answer.
(edit: However, there are errors in the questions in the Thom Nav book - and the others for that matter. It's been too long since I've used it to remember exactly what they are, but if a question really doesn't make sense Thom may have got it wrong).
Oh, and if you fly a Robin 2160, look at the compass when you turn the panel lights on. That's what your electrical systems do to your compass
