simplify -27 x^4 a^5 UNDER THE SQUARE ROOT SYMBOL THAT IS CUBED

I think you mean

∛(-27 x^4 a^5)
= ∛-27 ∛x^4 ∛a^5
= -3 x∛x a∛a^2
= -3ax∛(xa^2)

you could have typed

cbrt(-27 x^4 a^5)

or 4throot, 5throot, etc.