Answer :

3k^2 + 18k + 15 = 0
3k^2 + 18k = -15
k^2 + 6k = - 5
k^2 + 6k + 9 = -5 + 9
(k + 3)^2 = 4
k + 3 = (+-) sqrt 4
k = -3 (+-) sqrt 4

k = -3 + 2 = -1
k = -3 - 2 = -5

so k = -1 and -5

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