Can you plz help on this

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

Ann and betty together have $60 ann has $9 more than twice betty’s amount how much money dose each have

In a two-digit number the units' digit is 7 more than the tens' digit. the number with digits reversed is three times as large as the sum of the original number and the two digits. find the number.