A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 859248 can be evenly divided by 1 2 3 4 6 8 9 12 13 16 17 18 24 26 27 34 36 39 48 51 52 54 68 72 78 81 102 104 108 117 136 144 153 156 162 204 208 216 221 234 243 272 306 312 324 351 408 432 442 459 468 486 612 624 648 663 702 816 884 918 936 972 1,053 1,224 1,296 1,326 1,377 1,404 1,768 1,836 1,872 1,944 1,989 2,106 2,448 2,652 2,754 2,808 3,159 3,536 3,672 3,888 3,978 4,131 4,212 5,304 5,508 5,616 5,967 6,318 7,344 7,956 8,262 8,424 10,608 11,016 11,934 12,636 15,912 16,524 16,848 17,901 22,032 23,868 25,272 31,824 33,048 35,802 47,736 50,544 53,703 66,096 71,604 95,472 107,406 143,208 214,812 286,416 429,624 and 859,248, with no remainder.
Since 859,248 cannot be divided by just 1 and 859,248, it is not a prime number.