Does anyone know or understand how to do this?

If f(x)=3x+1 and f^-1=x-1/3, then f^-1(7)=

Liz is shipping a package to her friend. the maximum weight of a medium flat-rate box is 20 pounds, including the box which weighs 6 ounces. write and solve an inequality to describe the possible weight ,w, of the contents that liz will ship to her friend if she must use a medium flat-rate box.

Which of the following graphs could represent a cubic function?