Answer: 2
Each time x increases by 2, the outputs are multiplied by 4 (eg: 3 to 12 is times 4, or 12 to 48 is times 4)
Take the square root of 4 to get 2. So each time x increases by 1, y is doubled
x = 0, y = 3
x = 1, y = 6 <<-- y is doubled
x = 2, y = 12 <<-- y is doubled
and so on
