7 + 2/3(x) = -7
Since you can easily see that your end result is the opposite of one of the two parts on the opposite side of the equal sign, you at least know what your, in this case, middle number should be. Meaning, in order to get from 7 to -7 you know the number between them is 14 and that the 14 has to be negative because the number is being added to the +7 to get to -7.
What I would do, knowing this information, is multiply 14 by 3 (= 42) and then divide by 2 (21). Since you know it has to be negative you can just make the 21 negative. You can then substitute x for -21 and see that you get an answer of -7.
7 + 2/3(-21) = -7
7 + (2*-21)/3 = -7
7 + (-42)/3 = -7
7 - 14 = -7
-7 = -7
