Me i have been stuck on this question for 35 minutes

Find the nonpermissible replacement for binthis expression. b2/5b +5

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”?

Use order of operations to solve. (24 divided 3) + 3.5 + 3 power of 3