A polynomial of degree 5 would have a highest exponent of x equal to 5. So you have to cancel out the -3x6.
What would you add to -3x6 to cancel it out? Also, since there is not term with x ^ 5, you will also need to add a nonzero term with x ^ 5. (It could just be x ^ 5.) See if you can find the answer. You may submit your final answer to be checked by an expert.