Question 996952
You want an input of x instead of input of 2x-1.  Maybe the inverse of 2x-1 will be that input x.


{{{L(x)=2x-1}}}
{{{L^-1(x)}}} should be inverse of L.
{{{2*L^(-1)-1=x}}}
{{{2*L^(-1)=x+1}}}
{{{L^(-1)=(x+1)/2}}}


You want to form  {{{f(L(L^(-1)(x)))}}}
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This should be f(x):    {{{8((x+1)/2)^3 - 12((x+1)/2)^2 + 4(x+1)/2}}}


{{{(x+1)^3-3(x+1)^2+2(x+1)}}}
which is still much in unsimplified form.
You can do what you need or want with it, but that should be equivalent to f(x).