A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 409920 can be evenly divided by 1 2 3 4 5 6 7 8 10 12 14 15 16 20 21 24 28 30 32 35 40 42 48 56 60 61 64 70 80 84 96 105 112 120 122 140 160 168 183 192 210 224 240 244 280 305 320 336 366 420 427 448 480 488 560 610 672 732 840 854 915 960 976 1,120 1,220 1,281 1,344 1,464 1,680 1,708 1,830 1,952 2,135 2,240 2,440 2,562 2,928 3,360 3,416 3,660 3,904 4,270 4,880 5,124 5,856 6,405 6,720 6,832 7,320 8,540 9,760 10,248 11,712 12,810 13,664 14,640 17,080 19,520 20,496 25,620 27,328 29,280 34,160 40,992 51,240 58,560 68,320 81,984 102,480 136,640 204,960 and 409,920, with no remainder.
Since 409,920 cannot be divided by just 1 and 409,920, it is not a prime number.