The given equation is equivalent to quadratic equation

     = 0     (1)

with the leading coefficient 1.  (Notice that equation (1) is obtained from the given equation by division all the terms by 5.)


Therefore, equation (1) has the root z= 3.


According to Vieta's theorem, the product of the roots of the equation (1) is equal to its constant term, which is -6.

Therefore, the second root of the equation (1) is   = -2.


Thus the two roots of the equation (1) are 3 and -2.


Then, according to the Vieta's theorem, the sum of its roots (which is  3+(-2) = 1) is equal to the coefficient at x with the opposite sign.


In other words,   = 3+(-2) = 1,  which implies  k = -5.


ANSWER.  The second root of the given equation is -2 and k= -5.