it's easy!
let x1>x2>0,
f(x1)-f(x2)=f((x1/x2)*x2)-f(x2)
=f(x1/x2)+f(x2)-f(x2)
=f(x1/x2)
cause x1>x2>0,so x1/x2>1,so f(x1/x2)>0,
so f(x1)-f(x2)>0,that is f(x1)>f(x2)
f(x) is the increasing function when x>0
done
when you deal with a problem like this you should refer to the question and use the condition as possible as you can!