Asequence is defined by the recursive formula f (n + 1) = f(n) – 2. if f(1) = 18, what is f(5)?

Hello : 
this is  the arithmetic -  sequence when the common diference is : d = -2
the general term is : f(n) =f(1) +(n-1)d   .. d= -2  and f(1) =18
let : n= 5
f(5) = f(1) +(5-1)(-2)
f(5) = 18 -8 = 10

It’s hard to understand what you put.. can you show a picture ?