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