Reminder theorem:
Let p(x) be any polynomial of degree greater than or equal to one and let 'a' be any real number .if p(x) is divided by linear polynomial (x-a),then the remainder is p(a).
If there is a polynomial P(x) = a xⁿ + b xⁿ⁻¹ + ... + g x + h
Then the remainder of division of P(x) by (x - k) is given by P(x=k).
ie., remainder = a kⁿ + b kⁿ⁻¹ + .... + g * k + h
If P(k) = 0 , the n P(x) is divisible by (x - k)
Hope this helps :)