12, 9, 7, 3, 4, 6, ? which no replaces ?

Which Number Replaces the Question Mark (?) 12,9,7,3,4,6,?

Answer seems to be 9

See the explanation below

as number in upper line = 3(middle line) + opposite number in bottom line

Now how it is done

12 (The first number in upper line) = 3 (Middle line number) + ? (Opposite number in bottom line)

9 (The first number in upper line) = 3 (Middle line number) +6 (Opposite number in bottom line)

7 (The first number in upper line) = 3 (Middle line number) +4 (Opposite number in bottom line)

Which Number Replaces the Question Mark (?) 12,9,7,3,4,6,?