Can someone help me solve this

If f(x)=3x+1 and f^-1=x-1/3, then f^-1(7)=

Twelve less than a number is at most 20

Juanita wade's new car has an msrp of $28,902.11 including title and processing fees. the premier package which includes surround sound and dvd player costs three times the amount of the in-style package which includes leather seats and select wheels. the total cost of her new car was $34,290.08 which included the 6.5% sales tax. find the cost of the premier package to the nearest cent.