Hello!
Slope = 0.004
Vertical Intercept = 29.
Basically, you can rearrange this equation so that is in the form of y = mx + b.
y = mx + b
C(n) = 0.04n + 29
The slope represents the rate of change of the cost per minute talked, and the 29 (vertical intercept) represents the monthly fee. The monthly fee will stay the same, however, the number of minutes talked is susceptible to change, hence the variable n represents it.