A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 2521008 can be evenly divided by 1 2 3 4 6 7 8 9 12 14 16 18 21 24 28 36 41 42 48 56 61 63 72 82 84 112 122 123 126 144 164 168 183 244 246 252 287 328 336 366 369 427 488 492 504 549 574 656 732 738 854 861 976 984 1,008 1,098 1,148 1,281 1,464 1,476 1,708 1,722 1,968 2,196 2,296 2,501 2,562 2,583 2,928 2,952 3,416 3,444 3,843 4,392 4,592 5,002 5,124 5,166 5,904 6,832 6,888 7,503 7,686 8,784 10,004 10,248 10,332 13,776 15,006 15,372 17,507 20,008 20,496 20,664 22,509 30,012 30,744 35,014 40,016 41,328 45,018 52,521 60,024 61,488 70,028 90,036 105,042 120,048 140,056 157,563 180,072 210,084 280,112 315,126 360,144 420,168 630,252 840,336 1,260,504 and 2,521,008, with no remainder.
Since 2,521,008 cannot be divided by just 1 and 2,521,008, it is not a prime number.