i would do this
let
a = x ^ 3
da = 3x^2
hence the integral of
(x^5)(e^x^3)
becomes
(x^3)(x^2)(e^x^3)
u substitute everything in
[a(e^a)]/3
u use int by parts for this take out the 1/3 for this part
let u = a , du = da
v = e^a dv = e^a
so the integral is equal to uv - int(udv)
which is equal to
a(e^a) - int(e^a) = a(e^a) -e^a + C
= [x^3(e^x^3)-(e^x^3) ]/3 + C