In the middle of a test need help

How do you determine whether a relationship represented as as graph is linear or nonlinear

Aretha wanted to gather data about the cost of local bowling leagues in her area. she plotted the data and determined that the average bowling league costs consist of a one-time registration fee and a monthly fee modeled by the equation y = 15x + 20. identify and interpret the y-intercept in this model. the y-intercept is 20. this is the cost per month. the y-intercept is 20. this is the cost of registration. the y-intercept is 15. this is the cost of registration. the y-intercept is 15. this is the cost per month.

Sweet t saved 20 percent of the total cost of the green-eyed fleas new album let there be fleas on earth. if the regular price is $30 how much did sweet save