(1) Analyze du from the angle of bond breakage and formation; (2) There are two types of isomers zhi that meet the meaning of the title: one type is the benzene ring with 3 different substituents (hydroxyl, There are 10 isomers of carboxyl and methyl), and the other is 3 isomers with two substituents (hydroxyl and -CH 2 COOH) on the benzene ring. There are 10+3=13 kinds. (3) Design the synthetic route by reverse synthesis method. (4) Just use information for migration