A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 819840 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 96 105 112 120 122 128 140 160 168 183 192 210 224 240 244 280 305 320 336 366 384 420 427 448 480 488 560 610 640 672 732 840 854 896 915 960 976 1,120 1,220 1,281 1,344 1,464 1,680 1,708 1,830 1,920 1,952 2,135 2,240 2,440 2,562 2,688 2,928 3,360 3,416 3,660 3,904 4,270 4,480 4,880 5,124 5,856 6,405 6,720 6,832 7,320 7,808 8,540 9,760 10,248 11,712 12,810 13,440 13,664 14,640 17,080 19,520 20,496 23,424 25,620 27,328 29,280 34,160 39,040 40,992 51,240 54,656 58,560 68,320 81,984 102,480 117,120 136,640 163,968 204,960 273,280 409,920 and 819,840, with no remainder.
Since 819,840 cannot be divided by just 1 and 819,840, it is not a prime number.