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6

Are you sure

The pattern I saw was that the missing variable was the number one less than the first one indicated. 8(x7)=56, 7(x6)=42. With that reasoning, 3(x2)=6.

First you take square of a number then minus the same number from square result.
e.g 3*3 = 9
9-3 = 6
so answer is 6