To lease a mid-size sedan, a car dealership charges

$4000
up front and
$400
per month.

(a) Write a linear equation that represents the cost,
C
, of leasing the sedan for
x
months.
The constant cost is
$4000
. The variable cost is
$400
per month. If we replace
m
with
400
and
b
with
4000
, the equation is
C =