How can I find the compositon of ( f of g ) ?

Simply replace the x's in f(x) by 5/(x - 1) 

 

\(f(g(x))=\frac{5/(x-1)}{5/(x-1)+3}\rightarrow \frac{5}{5+3(x-1)}\rightarrow \frac{5}{3x+2}\)

 

.