what does x^2/3

In the manner you have it written, we have... (x^2) / 3

This says to square x and then divide by three.  So we have

(2)^2 / 3  =  4 / 3    And that's your answer      ....However.......

If you meant this,  x^(2/3), we have something different

This says to "square" x and then take the cube root of that answer

So we have 2^2 = 4

And the cube root of 4 can be written as (4)^(1/3) ≈ 1.59

Note that this brings up an important point......the placement of parentheses matter !!!! 

Hope that helped you