The reason they have to be simplified is that you cannot add radical unless the number under the radical is the same on all of them.
For example:
If you add √8 + √32 without simplifying, the best you could do is an estimate. But if you're required to put your answer in standard radical form this would be an issue.
Judging by the question I noticed that the two both share x^3 in common. You can divide x^4 by x^3 and get x as a result, and x^3 can still divide into itself.
