A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1398264 can be evenly divided by 1 2 3 4 6 7 8 12 14 21 24 28 29 41 42 49 56 58 82 84 87 98 116 123 147 164 168 174 196 203 232 246 287 294 328 348 392 406 492 574 588 609 696 812 861 984 1,148 1,176 1,189 1,218 1,421 1,624 1,722 2,009 2,296 2,378 2,436 2,842 3,444 3,567 4,018 4,263 4,756 4,872 5,684 6,027 6,888 7,134 8,036 8,323 8,526 9,512 11,368 12,054 14,268 16,072 16,646 17,052 24,108 24,969 28,536 33,292 34,104 48,216 49,938 58,261 66,584 99,876 116,522 174,783 199,752 233,044 349,566 466,088 699,132 and 1,398,264, with no remainder.
Since 1,398,264 cannot be divided by just 1 and 1,398,264, it is not a prime number.