To find a gene, you have to associate a phenotype with a genotype (e. g. all individuals with white flowers have a certain allele). it is much harder to find which genes are involved in polygenic traits than it is to find the genes responsible for discrete traits. why?

T's harder to find one of the many genes involved in a polygenic trait vs. the only gene involved in a discrete trait." 

It is probably not very difficult to find ONE OF THE GENES involved in a polygenic trait (as this answer suggests), but it IS difficult to find ALL OF THE GENES involved in the trait. 

This sounds like a high school Biology or first year university question - chances are they're testing your reading comprehension as much as your scientific knowledge.