habían adivinado
-they/you had guessed
Past perfect ellos/ellas/ustedes conjugation of adivinar.

adivinar

Que, por cierto, si no habían adivinado ya, es carbonato de calcio.
Oh, by the way, just in case you weren't already guessing, it's calcium carbonate.
Pero ya habían adivinado eso, ¿no?
But you'd guessed that already, hadn't you?
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