So.. what exactly IS the difference between these suffixes? Don't both sentences answer the same question (gdzie jest?)
The adjective has to agree in gender with the noun it describes, hence
we have "żółtej
", because "kanapa" is feminine, and "brązowym
"krzesło" is neuter.
If the couch was brown and the chair was yellow, we would have "brązowej
Simple as that ;)
Here's a useful site for declensions (it gives all possible declensions, without
describing their grammar usage though).