PLEASE HELP ME! I DONT UNDERSTAND

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”?

How can i differentiate this equation?

Zahra wants the equation below to have an infinite number of solutions when the missing number is placed in the box.