You need to notice that the members of fraction `(3+x)/(3x)` do not share duplicate factors, hence, the fraction is given in its simplest form.
You may perform a formal simplification if you write the fraction such that:
`((3 + x)*1)/((3x)*1)`
Reducing duplicate factor 1, yields:
`((3 + x)*1)/((3x)*1) = (3+x)/(3x) `
Hence, since the given form of fraction is its simplest form, it is not possible to be reduced to a simpler form than the given form.
I understand that you want to reduce to lowest terms. If it is about a fraction (since it is about reducing), it is obvious that the denominator and numerator are primes.
In other words, the numerator 3+x and the denominator 3x, have no common factors, it is impossible to reduce the fraction to the lowest terms.
(3+x)/3x cannot be reduced.