You are given two x and y pairs: (100,45) and (570,162.5)
First solve for the slope, m.
m = (162.5 - 45) / (570 - 100)
m = 117.5 / 470
m = 0.25
Not only is "m" the slope but it is the rate (dollars per minute) in the question. So it's 25 cents per minute to use the phone.
After imputing "m" the linear equation looks like this:
y = 0.25x + b
Use either point as (x,y) to solve for the y-intercept "b". I'll use (100,45)
45 = 0.25(100) + b
45 = 25 + b
45 - 25 = b
20 = b
Not only is "b" the y-intercept but it is also always the flat fee added on to linearly increasing cost questions like this.
Final equation is:
y = 0.25x + 20
Part B.) If 939 minutes are used. Just put that in for x and solve for y.
y = 0.25(939) + 20
y = 234.75 + 20
y = 254.75