Answer is 19,428.
DOM + all of the fuel is 43,372
MTOW is 62,800 for the MRJT, that leaves you with 19,428 for payload (allowable traffic load).
$$62,800 - (27,372 + 12,000 + 4,000)$$
Somewhere in the textbook the MTOW should be listed, the question as is now is unsolvable.
I got the 62,800 figure by Googling 'MRJT'.
