Solving a system of equations involving functions Solve this system of equations:
$$
\left\{\begin{array}{cccccr}
\displaystyle{\mathrm{f}\left(x\right)} & \displaystyle{+} &
\displaystyle{3\mathrm{f}\left(x - 1 \over x\right)} & \displaystyle{=} & 
\displaystyle{7x} & \displaystyle{\qquad\qquad\qquad\qquad\left(\mathrm{A}\right)}
\\
\displaystyle{\mathrm{f}\left(x - 1 \over x\right)} & \displaystyle{+} &
\displaystyle{3\mathrm{f}\left(1 \over 1 - x\right)} & \displaystyle{=} & 
\displaystyle{7x - 7 \over x} & \displaystyle{\left(\mathrm{B}\right)}
\\
\displaystyle{\mathrm{f}\left(1 \over 1 - x\right)} & \displaystyle{+} &
\displaystyle{3\mathrm{f}\left(x\right)} & \displaystyle{=} & 
\displaystyle{7 \over 1 - x} & \displaystyle{\left(\mathrm{C}\right)}
\end{array}\right.
$$
I've never solved a system of equations with functions, so I'm brand new to this concept. Could someone clue me out or provide a solution ?. Thanks !.

When given such a system of equations, you are expected to find a function (here $f$) that satisfies all those equations for all values of $x$.
Let $\frac{x-1}x=y$ and $\frac1{1-x}=z$. Then
$$f(x)+3f(y)=7x\tag1$$
$$f(y)+3f(z)=7y\tag2$$
$$f(z)+3f(x)=7z\tag3$$
Eliminate $f(z)$ by subtracting $(3)$ from $(2)$ thrice:
$$f(y)-9f(x)=7y-21z\tag4$$
Eliminate $f(y)$ by subtracting $(4)$ from $(1)$ thrice:
$$f(x)+27f(x)=28f(x)=7x-21y+63z$$
$$4f(x)=x-3y+9z$$
Hence we have the desired expression for $f(x)$:
$$f(x)=\frac14\left(x-\frac{3(x-1)}x+\frac9{1-x}\right)$$
$$=\frac{x^2(1-x)-3(x-1)(1-x)+9x}{4x(1-x)}$$
$$=\frac{x^2-x^3+3-6x+3x^2+9x}{4x(1-x)}$$
$$=\frac{-x^3+4x^2+3x+3}{4x(1-x)}$$
