Ok so I think the answer is F(1)=31. And if u need to show work then change the equation to f(n-1)=f(n)+3 and substitute the f(n)'s. For example, to get f(3) you must substitute it like this: f(4-1)=(22)+3 and then f(3) = 25. And just keep doing that until you get f(1). I hope the answer is right.