Eva is a kickboxing instructor who will be teaching classes at a local gym. to get certified as an instructor, she spent a total of $57. eva will be earning a base salary of $51 per month from the gym, plus an additional $3 for every class she teaches. if eva teaches a certain number of classes during her first month as an instructor, she will earn back the amount she spent on certification. how much will eva's expenses and earnings be? how many classes will that take?