Avery is moving and must rent a truck. There is an initial charge for the rental plus a fee per mile driven. Let C represent the total cost of renting the truck when the truck is driven m miles. The table below has select values showing the linear relationship between m and C. Determine the cost per mile that Avery drives.

M: 2, 4.5 ,7
C: 46, 53.5 , 61