A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 399168 can be evenly divided by 1 2 3 4 6 7 8 9 11 12 14 16 18 21 22 24 27 28 32 33 36 42 44 48 54 56 63 64 66 72 77 81 84 88 96 99 108 112 126 132 144 154 162 168 176 189 192 198 216 224 231 252 264 288 297 308 324 336 352 378 396 432 448 462 504 528 567 576 594 616 648 672 693 704 756 792 864 891 924 1,008 1,056 1,134 1,188 1,232 1,296 1,344 1,386 1,512 1,584 1,728 1,782 1,848 2,016 2,079 2,112 2,268 2,376 2,464 2,592 2,772 3,024 3,168 3,564 3,696 4,032 4,158 4,536 4,752 4,928 5,184 5,544 6,048 6,237 6,336 7,128 7,392 8,316 9,072 9,504 11,088 12,096 12,474 14,256 14,784 16,632 18,144 19,008 22,176 24,948 28,512 33,264 36,288 44,352 49,896 57,024 66,528 99,792 133,056 199,584 and 399,168, with no remainder.
Since 399,168 cannot be divided by just 1 and 399,168, it is not a prime number.