If a polynomial p(x) is divisible by q(x), the roots of q(x) are roots of p(x), too.
In this question, p(x)=x³+6x²+kx+12 and q(x)=x+4. Clearly, the root of q(x) is -4. By what was written initially, -4 is a root of p(x). So:
In this question, p(x)=x³+6x²+kx+12 and q(x)=x+4. Clearly, the root of q(x) is -4. By what was written initially, -4 is a root of p(x). So: