A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 891072 can be evenly divided by 1 2 3 4 6 7 8 9 12 13 14 16 17 18 21 24 26 28 32 34 36 39 42 48 51 52 56 63 64 68 72 78 84 91 96 102 104 112 117 119 126 136 144 153 156 168 182 192 204 208 221 224 234 238 252 272 273 288 306 312 336 357 364 408 416 442 448 468 476 504 544 546 576 612 624 663 672 714 728 816 819 832 884 936 952 1,008 1,071 1,088 1,092 1,224 1,248 1,326 1,344 1,428 1,456 1,547 1,632 1,638 1,768 1,872 1,904 1,989 2,016 2,142 2,184 2,448 2,496 2,652 2,856 2,912 3,094 3,264 3,276 3,536 3,744 3,808 3,978 4,032 4,284 4,368 4,641 4,896 5,304 5,712 5,824 6,188 6,552 7,072 7,488 7,616 7,956 8,568 8,736 9,282 9,792 10,608 11,424 12,376 13,104 13,923 14,144 15,912 17,136 17,472 18,564 21,216 22,848 24,752 26,208 27,846 31,824 34,272 37,128 42,432 49,504 52,416 55,692 63,648 68,544 74,256 99,008 111,384 127,296 148,512 222,768 297,024 445,536 and 891,072, with no remainder.
Since 891,072 cannot be divided by just 1 and 891,072, it is not a prime number.