I assume that you want to subtract the value at -1 from the value at 1.
Well, the function is an even function so the answer is zero.
This looks like you actually are doing a calculus problem to integrate this function from -1 to 1:
x/3 + (x^3)/2
And, again, since you are integrating an odd function you will immediately answer that the result is zero.
But if you want to use Wolfram|Alpha (rather than thinking the problem through first!) the instruction might be
integrate x/3 + (x^3)/2 from -1 to 1