rule=
input3 output 2
input 6 output3
input 9 output4
input 12 output 5
output = 1/3 input + 1
For stuff like this, watch to see how each side changes. The output increased by 1 when the input increased by 3.
So, the output is 1/3 the input, with an offset of some kind. In this case, +1.