I dont know what the question is but if the question is to solve this eq. then the method will be as given below

tan²x tan²3x tan4x = tan²x  - tan²3x + tan4x

so tan4x =  [tan²x  - tan²3x] / [tan²x tan²3x-1]

now split RHS terms by formula x²-y²= (x+y)(x-y)

so RHS = tan4xtan2x 

now solve