I found this in a 2013 MAT question whilst practising past papers and am not sure of the meaning of this notation. Keep in mind my understanding of maths is only at the level of Scottish Highers (similar to A level maths).
question iii) asks about $f_k(x)$ and $f_{2-k}(x)$, what is the difference?
The answers to question i) also reference a polynomial $p_3$ where 3 is the highest index of the polynomial, is this also what is meant by the notation for $f_k(x)$?
The question for context
$f_k$ is just used to denote a family of functions. It's just like you define a sequence $a_k=2k-1$, which gives all odd numbers. You put a value of $k$, say $7$ and get back the value of $a_7$. Here, $k$ is just a parameter. In your case for example, you will have $$f_7(x)=x(x-7)(x-2)$$ $$f_{2-k}(x)=x\Big(x-(2-k)\Big)(x-7)=x(x+k-2)(x-7)$$ Hope this helps. Ask anything if not clear :)