This is a jukugo word that uses the on'yomi readings of the kanji. You haven't learned the on'yomi for the second kanji, but if you remember the meaning of the word, that will get you to the reading. In case you're having trouble, here's a mnemonic to help:
You sit eating senbei next to a bay (べい), looking out over the water as you crunch.
These are rice crackers, a souvenir from Tokyo.
Green tea always goes well with senbei.
Viet got annoyed and threw the rice crackers.