12.
You convert the letters into numbers (A=1, B=2, etc). You then sum the numbers of each set. The difference is the number in the corner.
MFK = 13 + 6 + 11 = 30, NRG = 14 + 18 + 7 = 39
NRG - MFK = 39 - 30 = 9
DPG = 4 + 16 + 7 = 27
NRG - DPG = 39 - 27 = 12
