Topic maths series? Need help

find the of the term which is independent of x in the expansion of (x+3/x)^4

please explain this

The expansion of (a+b)^4 has terms of the form (4Cr)x^r(3/x)^(4-r)
and to be independent of x , x^r=x^(4-r) so r=2 and the required term
is 4C2(3^2)=54