calculus 1 help please! word problem (cost of production/derivative)

(Wondering)" this project is an application of derivatives to cost and fuel efficiency for your vehicle"

the average yearly mileage for my car to the nearest thousand is 7000 miles (from question #1)...

in question #2 i had to state the average price per gallon that i pay for gas which came to be $2.87/gallon

now i have to write the function C(x) which is the annual cost of fuel for my vehicle as a function of the miles per gallon x. (do not use a specific number for miles per gallon for x here) show work

(Crying)help me with this last question ..i dnt know what to do!!!

(my car has a fuel capacity of 20 mpg...and i use 23 miles (from my house to school) as a reference to get the 7000 miles in question #1 maybe this can help ..i dnt know!!)