a taxi company charges passengers $2.00 for a ride, no matter how long the ride is, and an additional $0.20 for each mile traveled. the rule c=0.20m + 2.00 describe the relationship between the number of miles m and the total cost of the ride c.
The cost of the ride varies by however many miles is driven, however the charging rate stays the same no matter how long the ride is. In the expression 0.20m + 2.00 , 2.00 is the constant as it stays the same, and 0.20 is the coefficient as is varies with however many miles are driven.
