Answer
$f(g(x))=x,$
$g(f(x))=x$
Work Step by Step
$f(x)=\frac{x-5}{2x+3},$
$g(x)=\frac{3x+5}{1-2x},$
$f(g(x))=\frac{(\frac{3x+5}{1-2x})-5}{2(\frac{3x+5}{1-2x})+3}=\frac{13x}{13}=x,$
Domain of $f(g(x))$ is, $x \in \mathbb{R} \ne \{ \frac{1}{2}\}$
$g(f(x))=\frac{2(\frac{x-5}{2x+3})+5}{1-2(\frac{x-5}{2x+3})}=\frac{13x}{13}=x$
Domain of $f(g(x))$ is, $x \in \mathbb{R} \ne \{ -\frac{3}{2}\}$