A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 815724 can be evenly divided by 1 2 3 4 6 7 9 12 13 14 18 21 26 27 28 36 39 42 52 54 63 78 83 84 91 108 117 126 156 166 182 189 234 249 252 273 332 351 364 378 468 498 546 581 702 747 756 819 996 1,079 1,092 1,162 1,404 1,494 1,638 1,743 2,158 2,241 2,324 2,457 2,988 3,237 3,276 3,486 4,316 4,482 4,914 5,229 6,474 6,972 7,553 8,964 9,711 9,828 10,458 12,948 15,106 15,687 19,422 20,916 22,659 29,133 30,212 31,374 38,844 45,318 58,266 62,748 67,977 90,636 116,532 135,954 203,931 271,908 407,862 and 815,724, with no remainder.
Since 815,724 cannot be divided by just 1 and 815,724, it is not a prime number.