Translation:Eighteen black pants, not nineteen.
I wrote "pairs of pants" and got marked wrong, but surely that should be accepted, yes? Whenever I encounter this garment as a countable noun, it's always "pairs of pants." "Pants" by itself is only ever used either when talking about a singular garment, or the uncountable concept of pants in general.
Maybe it's a regional thing?
If you have an indefinite noun with an adjective, it means the adjective is descriptive. מכנסיים שחורים "black pants".
If there is a copula, it means the adjective is a predicate adjective, and the copula stands for English "is" or "are". מכנסיים הם שחורים - pants are black.