Given Word Addressable (means every word can be identified uniquely)
given :segment table is divided into 512 pages and each page contain 1024 entries
but each page table length is (2word) therefore 2^9 indexing must be there to identify each word uniquely
therefore no of bits for indexing page table is 9 means 2^9=512
and 512 *2words =1024 words is the size of page table
may be ,i too wrong ,but if u find the reason please share