You can rewrite as a sequence of values (the outputs): 2,5 ,8 , 11, ....
for the input 5, we must to have the output 14 (look that the sequence above, is take +3 every time)
So, for the input n, we must have
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Why?
Look that, for n = 1, we need to have 2, and for n=2 we need to have 5, that correspond to take +3 for the predecessor...