Question 776720
x is for miles per month, a variable.
m is used with x in the manner, mx = the cost according to miles per month.
If you want y to be the monthly cost for using the car, then the next cost contribution is the fixed cost of $172.90.  
The meaning for m is the $0.34 per mile.


Summary:  Variable cost is{{{ mx=0.34*x}}}
Fixed cost is ${{{172.80}}}


Using y as the monthly cost to operate this car, 
{{{highlight(y=0.34x+172.90)}}}
The b from general form of the linear equation is the y intercept.  When the car is not used in the one month period, the cost is still at least that value, {{{b=172.90}}}.