Very simple.Just use the law of equivalents.Remember Equivalents or milliequivalents react in equal number to give equal number 0f equivalents of Products.
Since 39mg of Al[OH]3 reacts completely with the amount present of [COOH]2.
So milliequivalents of [COOH]2=Milliequivalents of AL[OH]3
Equivalent weight of Al[OH]3 is M/3
So 39/78 x 3=20xN [Milliequivalents=NV,where v in ml]
And you already have calculated the Equivalent weight of [COOH]2.Its M/2