I think it must be 3x^3 - 20x^2 + kx + 12 ????
3 is one root
3 [ 3 - 20 k 12 ]
9 -33 3k - 99
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3 -11 k - 33 3k - 87
3k - 87 = 0
3k = 87
k = 29
3 [ 3 -20 29 12 ]
9 -33 -12
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3 -11 -4 0
The remaining polynomial is
3x^2 -11x - 4 = 0
(3x + 1) ( x - 4) = 0
The other two roots are -1/3 and 4