Answer:
f(x) = x * -3
Explanation:
so, you have x and you have f(x), as you can see -4 turns into 12... you basically have to divide, multiply, add or subtract for each value to see what they have in common, here it's easy since -4 * -3 is 12, -2 * -3 is 6, 0 * -3 is 0, etc etc. hope it helped : )