A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 2589888 can be evenly divided by 1 2 3 4 6 7 8 12 14 16 21 24 28 32 41 42 47 48 56 64 82 84 94 96 112 123 141 164 168 188 192 224 246 282 287 328 329 336 376 448 492 564 574 656 658 672 752 861 984 987 1,128 1,148 1,312 1,316 1,344 1,504 1,722 1,927 1,968 1,974 2,256 2,296 2,624 2,632 3,008 3,444 3,854 3,936 3,948 4,512 4,592 5,264 5,781 6,888 7,708 7,872 7,896 9,024 9,184 10,528 11,562 13,489 13,776 15,416 15,792 18,368 21,056 23,124 26,978 27,552 30,832 31,584 40,467 46,248 53,956 55,104 61,664 63,168 80,934 92,496 107,912 123,328 161,868 184,992 215,824 323,736 369,984 431,648 647,472 863,296 1,294,944 and 2,589,888, with no remainder.
Since 2,589,888 cannot be divided by just 1 and 2,589,888, it is not a prime number.