The problems in the middle

One line passes through (-7,-4) and (5,4) . another line passes through the point (-4,6) and (6,-9)

Having a hard time understanding this question (see attachment).

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”?