Please help I think its 16 but not sure

Show algebraically that f and g are inverse functions f(x)=x^2/-6; g(x)=square root of 2x+12

The distribution of the amount of money spent by students for textbooks in a semester is approximately normal in shape with a mean of $235 and a standard deviation of $20. according to the standard deviation rule, how much did almost all (99.7%) of the students spend on textbooks in a semester?

Hi i’m not sure how to do question 20 if u could explain how to do it that’d b great