確か
たしか · tashika · JLPT N4 · na-adj
certain
確 = firm, verified; 確か is the adjective: certain, or as far as I recall
確 pairs 石 (stone) with a phonetic part, giving the sense of rock-solid. The same kanji appears in 確認 (confirm) and 確実 (reliable). Used alone at the start of a sentence, たしか softens to: if I remember correctly.