Changes in output are evenly spaced for evenly spaced changes in input, so the rule is linear. Changes in output are 1 for changes in input of 3, so the rule involves a factor of 1/3. One possible rule is
... output = (1/3)·input + 3
If y and x are used to represent output and input, respectively, this is
... y = (1/3)x + 3
