This question tests on stoichiometry ratios, in which NaOH is used to completely neutralised by NaOH. Since oxalic acid is a dicarboxylic acid (acid having 2 COOH) groups, the stoichiometry ratio of NaOH: Oxalic Acid is 2:1. Since we can calculate immediately the amount of NaOH used to neutralise both acids and the amount of ethanoic acid present, we can calculate the amount of NaOH used to neutralise ethanoic acid and hence the amount used to neutralise oxalic acid. From there, further use stoichiometry to calculate the amount of oxalic acid present and hence its concentration. Hope this helps!