GMAC practice test - tough one

[Striver]

From GMAC practice test:

For which of the following functions is f(a + b) = f(a) + f(b) for all positive numbers a and b?

The answer to this was f(x) = -3x. I don't understand why?

[GMATGuruNY]

I posted an explanation here:

https://www.beatthegmat.com/gmat-prep-fu ... 67229.html

[Anurag@Gurome]

Refer to the post here >> https://www.beatthegmat.com/functions-t1 ... tml#477877