Here Page size =1 KB =1024B =2^10 B =2^13 bits
Page table entry size = 4 B = 32bits =2^5 bits
No of page table entries in outer page table that fits in one page = 1024 B / 4B = 2^8 entries
So this entries will point to one page in 2nd page table
Total no of entries in 2nd page table = 2^8 * 2^ 8 = 2^16
Total no of entries in 3rd page table = 2^8 * 2^ 8 *2^8 = 2^24
these entries will point to no of pages in virtual memory
there fore virtual memory size = 2^24 pages * 1024 B (page size) = 2^34 B = 2^37 bits =137438953472 bits