Learn Russian: Твои чёрные брюки здесь.

Because the word for trousers, брюки, is plural. In Russian, possessive words like "your" must match the gender and number of the noun they belong to. Твои is the plural form of the informal "your".

Why does the adjective чёрные end in -ые?

Adjectives must also match the noun they describe. Since брюки is plural, the adjective takes the standard nominative plural ending -ые instead of a singular masculine ending like -ый.

Is брюки always plural?

Yes! Just like the words "trousers" or "pants" in English, брюки is always a plural word in Russian. It has no singular form, which is why everything describing it in this sentence must also be plural.

Why isn't there a word for "are" in this sentence?

In Russian, the present tense of the verb "to be" (am, is, are) is almost always dropped. You just say "Your black trousers here," and the "are" is naturally understood.