Given a Word Problem, Write an Equation in Slope-Intercept Form (y=mx+b)

The problem:
Jack loves his i-phone. He had to pay a certain amount for his phone upfront, and then a monthly fee to use the phone. After 10 months, Jack has paid a total of $850. After 12 months, Jack has paid $980.
a. Write an equation, in y = mx + b form, that represents this situation.

b. In your equation, what does the y represent?

c. What is the monthly fee to use the phone?

d. How much will it cost Jack to own his phone for 18 months?

The solution:
a. y=65x+200; x=months and y=cost
b.How much Jack has paid after x months.
c.$65 per month
d. 18 months.

[redopps]

Thanks I was really struggling before the video!