The circumference of a circle is C = 2πr^2 (2nd power) Radius (r) is the distance from the center of the circle to the outside of it. To find radius divide the diameter (d) by 2. The diameter (d) of a circle is the radius (r) multiplied by 2. To find the diameter (d) you need to do C ÷ π.
For example): Finding diameter - 76/3.14 = 24.19 (d)
Since there is a flat rate of 15, that is your y-intercept. The $3 charge is variable and depends on the number of days, so that would represent your slope.