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Mathematical problem box
Because the sum of the numbers on the three lines is equal, the sum of the three lines is a multiple of 3. The number represented by a has been calculated here three times.

( 1+7)*7/2=28

28 plus twice 1-7 is a multiple of 3, which may be 1, 4, 7.

So the letter a may be 1, 4, 7.