how is the following simplified

Question

3x to the third power plus 5x to the fifth power

Answer 1

I don't know whether you mean
(3x)^3 + (5x)^5, or
3x^3 + 5x^5.

You could write the first version as

27x^3 [1 + (125/27)x^2]

but that would not really be any simpler.