(i) Solution: $ x(3) = x(2) + 2u(2) = x(1) + 2u(1) + 2u(2) = x(0) + 2u(0) + 2u(1) + 2u(2) $
$ \because x(0) = 3, x(3) = 9 $
$ \therefore \ Constraint: 2u(0) + 2u(1) + 2u(2) = 6 $
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