This is the 1277th most frequent Turkish word.
bulduğu
"Bulduğu" means "that he/she/it found" or "what he/she/it found."
The word 'bulduğu' (he/she found) is used as the complement of the relative clause modifying the noun 'kitabı' (the book).
O, bulduğu kitabı arkadaşına gösterdi.
He showed the book he found to his friend.
Here, 'bulduğu' describes the money that has been found, shaping the relative clause.
Adam bulduğu parayı cüzdanına koydu.
The man put the money he found into his wallet.
In this sentence, 'bulduğu' refers to the solutions and creates a relative phrase modifying 'çözümleri' (solutions).