たしか
たしか · tashika · JLPT N3 · na-adj
sure
確か: 確 = firm, solid, the same kanji as 確認 kakunin, to confirm
たしか is written 確か, and 確 means firm or solid, so the word means something stands firm: certain or sure. As an opening word, たしか… softens this to 'if I remember correctly', meaning the speaker thinks the fact is firm.