For point a, you know that C(n) = 31 so you can plug this value into the equation and solve for n, like this
Therefore, the value of n such that C (n) = 31 is true is n = 9.
For the point b, since the given function corresponds to a cafeteria meal plan based on the number of meals purchased, n, then a value of 9 for n indicates that a meal plan that has 9 meals will be worth $ 31.