Hello Gmat_mission.

Let's take a look at your question.

We need to answer is a is positive or not.

First Statement

(1) a - a = 0

This statement doesn't tell us anything about a, because $$a-a=0\ \ \ \Rightarrow\ \ 0=0.$$ Then, we don't know anything about a. Not Sufficient.

Second Statement

(2) a + a = 2a

This statement doesn't tell us anything about a, because $$a+a=2a\ \ \ \Rightarrow\ \ 2a=2a\ \Rightarrow\ \ 0=0.$$ Then, we don't know anything about a. Not Sufficient.

First Statement+ Second Statement

(1) a - a = 0
(2) a + a = 2a

Again, none of the statements give us information about a. Not Sufficient.

Therefore, the correct answer is the option E.

I hope it is clear enough.