Writing a chemical equation with a reactant of unspecified chain length
Tips and hints would be greatly appreciated.

My tips and hints for you (I apologize for if my tips and hints direct to me treating OP as a novice):

Your dibasic acid is $\ce{HO2C-(CH2)n-CO2H}$. Thus, $\ce{A^2-}$ in the equation $(2)$ of Karsten Theis's answer is $\ce{^-OOC-(CH2)n-COO^-}$.
Molar mass of this dibasic acid is: $2 \times (12+32+1) + 14n = 90 + 14n$
Find the concentration of FA2 solution in $\pu{mol dm-3}$ (in other words, in $\pu{M}$. Hence, you can find the the amount of $\ce{NaOH}$ (in $\pu{mol}$) in FA2 solution ($\pu{17.0 cm3}$).
Determine the amount of dibasic acid (in $m_\ce{H2A} \ \pu{ mol}$) in FA1 solution ($\pu{25.0 cm3}$) using the equation $(2)$ of Karsten Theis's answer.
Now, you can find $n$ using following equation:
$$m_\ce{H2A} \ \pu{mol} \times \pu{(90 + 14n) g mol-1} = \pu{5.00 g dm-3} \times \pu{0.025 dm-3}$$

Since you have already calculated $m_\ce{H2A}$ in step 4, you can calculate the $n$ $(\pu{1.0 dm-3} = \pu{1000 cm-3})$.
Note that $n$ is an integer so final answer should be rounded to closest integer.