Find the inverse of the function

f(x) = x / 2x+1

I know that you would make this
x= y / 2x+1 but now I do not know what to do.

no, not exactly.

y=x/(2x+1)
2xy+y=x
x(2y-1)=-y I skipped a step here, work it out

2xy-x=-y
x(2y-1)=-y
x=-y/(2y-1)

so the inverse function is

f'(x)=-x/(2x-1)