First, simplify the question:
Is ax + bx = 6 ?
Is x(a + b) = 6 ?
Either of those versions could turn out to be helpful.

Statement (1) is insufficient. We can simplify the question further:
Is ax + ax = 6 ?
Is 2ax = 6 ?
Is ax = 3 ?

Statement (2) is also insufficient. With this statement alone, we don't know anything about b, so the value of ax isn't enough.

Taken together, the statements are sufficient. Statement (1) tells us that the question is identical to:
Is 2ax = 6 ?
and (2) tells us that this is true: 2ax = 6. Choice (C) is correct.