Sistemas de ecuaciones no lineales

Ineed big ! the cost of a school banquet is $25 plus $15 for each person attending. create a table, sketch the graph, and write an equation in slope-intercept and point-slope form that gives total cost as a function of the number of people attending. what is the cost for 77 people?

Math i have no idea how to do a and b

Find the value of x. also give me an explanation so i can complete it to other questions as well, : )