2.。 . . c4 1+c42+c43= 14

3.。 . . k=5，(x？ +4x-5)(x？ -6x+k)=0 is x? +4x-5=0 or x? -6x+k=0, and the last solution is x= 1 or -5.

And because {-5, 1, 5}, X should also have a solution of 5. Substitute x=5 for X? -6x+k=0, and the solution is k=5.