On field trips, there must be 2 chaperones for every 15 students. which ratio correctly expresses the ratio of students to chaperones? 2: 15 15: 17 17: 15 15: 2

15 students to 2 chaperones is expressed by the student to chaperone ratio
  15 : 2

Abit confused about what you mean, but i think you might be referring to slope intercept form: y = mx + bin that case b, or the y intercept is already given.(0,-1)find the slope by using the formulay2 - y1/x2 - x1the final slope will be 4/2, equivalent to 2.now your equation should look like thisy = 4/2x -1sorry if this didn’t you out!