A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 2558976 can be evenly divided by 1 2 3 4 6 7 8 12 14 16 17 21 24 28 32 34 42 48 49 51 56 64 68 84 96 98 102 112 119 128 136 147 168 192 196 204 224 238 256 272 294 336 357 384 392 408 448 476 512 544 588 672 714 768 784 816 833 896 952 1,024 1,088 1,176 1,344 1,428 1,536 1,568 1,632 1,666 1,792 1,904 2,176 2,352 2,499 2,688 2,856 3,072 3,136 3,264 3,332 3,584 3,808 4,352 4,704 4,998 5,376 5,712 6,272 6,528 6,664 7,168 7,616 8,704 9,408 9,996 10,752 11,424 12,544 13,056 13,328 15,232 17,408 18,816 19,992 21,504 22,848 25,088 26,112 26,656 30,464 37,632 39,984 45,696 50,176 52,224 53,312 60,928 75,264 79,968 91,392 106,624 121,856 150,528 159,936 182,784 213,248 319,872 365,568 426,496 639,744 852,992 1,279,488 and 2,558,976, with no remainder.
Since 2,558,976 cannot be divided by just 1 and 2,558,976, it is not a prime number.