Asequence is defined by the recursive function f(n + 1) = 1/2f(n). if f(3) = 9 , what is f(1) ?

To find f(1) it might be a good idea to see what the problem is actually telling us.  if f(3) = 9... if we put that into the original function that means that
f(3 + 1) = 1/2f(3) so f(4) = 1/2f(3) since f(3)=9 then f(4) = 4.5... so it is one half of f(3)... now to get to f(1) you need to do the opposite.  f(2) will be twice f(3) and so on.
f(4) = 4.5
f(3) = 9
f(2) = 18
f(1) = 36

so f(1) = 36