This is the 595th most frequent Persian word.
بزرگترین
The greatest.
Here, "بزرگترین" modifies "مشکل" (problem), indicating the greatest or most significant one among a set of potential problems.
بزرگترین مشکل من پیدا کردن وقت آزاد است.
My biggest problem is finding free time.
In this sentence, "بزرگترین" describes "شهر" (city), specifying it as the largest in size or population in the country.
بزرگترین شهر ایران تهران است.
The biggest city in Iran is Tehran.
Here, "بزرگترین" relates to "دانش آموز" (student), referring to being the oldest in terms of age within a group.