How do i solve this type of problem?

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

For what value of (y) is 3^3/3^y =1/9

Tabby sells clothing at a retail store, and she earns a commission of 6.25% on all items she sells. last week she sold a pair of jeans for $32.50, a skirt for $18.95, and 2 blouses for $14.95 each. what was tabby's total commission? a $4.15 b $5.08 c $81.35 d $348.60