because Pantalones is plural. donde están las flores = where are the flowers? (plural). donde está ella = where is she? donde estais/estas = where are you? i hope you can understand :D

Pantalones és una palabra que está siempre en el plural. Por eso hay que usar siempre los verbos conjugados en el plural.

since I am a beginner in learning spanish , I am not sure about my answer... :P but I'd like to think it as "sus pantalones están aquí", then "sus pantalones" is the subject, and it is the third-person plural, so we use "están" I guess it's like"here comes it" in English. "it" is the subject

pantalones = third person plural Therefore you need "están".