Ah, BOAC got in first!
It's a bit of an odd question with some very inexact wording.
It would be better if they used a term like Dry Operating Mass (DOM) instead of 'empty weight'. 'Empty Weight' tends to mean 'the weight of the aircraft with only unusable fuel and fluids in sealed systems on board, no crew and no catering equipment etc.', items sometimes called the 'variable load'. The variable load would not count as 'freight'. DOM = 'empty weight' plus variable load.
Assuming that the 'ZFM' in the question actually means MZFM, it would be true to say that the maximum freight limited by ZFM would be MZFM - DOM, in this case 17t - 12t = 5t.
If we continue with that assumption then there is no need to assume that all the freight is carried, the actual ZFM could be less than 17t and more than 4t of fuel could be carried. If 'ZFM' means 'actual ZFM' you are right, you can only carry 4t of fuel.
What is really odd is that a DOM of 12t and a MTOM of 21t only allows a fuel load of 9t, and that assumes no freight at all. It's difficult to see how a plane could burn 10t if it could only carry 9 on take-off.
Where did this come from?