In order to solve this problem, we need to use the factor and remainder theorem. Suppose we have a quadratic equation of Q(x) and a binomial x-a=0, where a is a constant such that x=a. When you substitute x=a to the quadratic equation, you use the answer as basis. Whatever the answer is, is the remainder. If the answer is zero, that means that x=a is a factor.
In this case, we substitute x=3 to the quadratic equation and equate to zero to find k.
