f(n) = f(n+1) +3
f(1) = f(2) +3
f(2) =f(3) +3
f(3) = f(4) +3
So f(1) = f(n) + 3* (n-1)
Put n=4 here so you found
f(1) = f(4) + 3* 3
= 22 +9
=31 ans
hope it helps
