answer: In the picture, the square of the difference of the two numbers above plus the square of the difference of the two numbers below will get the middle number.    We have: (7 - 4) ^ 2 + (8 - 3) ^ 2 = 34.    (5 - 1) ^ 2 + (6 - 4) ^ 2 = 20.    So: (9 - 3) ^ 2 + (4 - 2) ^ 2 = 40.