$11$ Bargain Rental Car wants to better understand the variable and fixed portions of its car washing costs. The company operates its own car wash facility that cleans each rental car before releasing it to another customer. Management believes the variable portion of its car washing costs relates to the number of rental returns. Accordingly, the following data have been compiled:

$\backslash$table$[[$Month$,$Rental Returns,Car Wash Costs$],[$January$,2,400,$$$10,500$