A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1639680 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 96 105 112 120 122 128 140 160 168 183 192 210 224 240 244 256 280 305 320 336 366 384 420 427 448 480 488 560 610 640 672 732 768 840 854 896 915 960 976 1,120 1,220 1,280 1,281 1,344 1,464 1,680 1,708 1,792 1,830 1,920 1,952 2,135 2,240 2,440 2,562 2,688 2,928 3,360 3,416 3,660 3,840 3,904 4,270 4,480 4,880 5,124 5,376 5,856 6,405 6,720 6,832 7,320 7,808 8,540 8,960 9,760 10,248 11,712 12,810 13,440 13,664 14,640 15,616 17,080 19,520 20,496 23,424 25,620 26,880 27,328 29,280 34,160 39,040 40,992 46,848 51,240 54,656 58,560 68,320 78,080 81,984 102,480 109,312 117,120 136,640 163,968 204,960 234,240 273,280 327,936 409,920 546,560 819,840 and 1,639,680, with no remainder.
Since 1,639,680 cannot be divided by just 1 and 1,639,680, it is not a prime number.