• f'(x) = 2X - 6k - 18 < 0
    = 2X < 6k - 18
    = X < 3k + 9

    -3 < 3k + 9 < 2
    -12 < 3k < -7
    -12/3 < k < -7/3
    -4 < k < -7/3

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