Someone help me with 5 and 6 please I have to finish this RN PLEASE THANK U

If you could answer these your a life saver

Scores on a university exam are normally distributed with a mean of 78 and a standard deviation of 8. the professor teaching the class declares that a score of 70 or higher is required for a grade of at least “c.” using the 68-95-99.7 rule, what percentage of students failed to earn a grade of at least “c”?

Factor the polynomial expression x6 – x3 – 20