A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1644720 can be evenly divided by 1 2 3 4 5 6 7 8 10 11 12 14 15 16 20 21 22 24 28 30 33 35 40 42 44 48 55 56 60 66 70 77 80 84 88 89 105 110 112 120 132 140 154 165 168 176 178 210 220 231 240 264 267 280 308 330 336 356 385 420 440 445 462 528 534 560 616 623 660 712 770 840 880 890 924 979 1,068 1,155 1,232 1,246 1,320 1,335 1,424 1,540 1,680 1,780 1,848 1,869 1,958 2,136 2,310 2,492 2,640 2,670 2,937 3,080 3,115 3,560 3,696 3,738 3,916 4,272 4,620 4,895 4,984 5,340 5,874 6,160 6,230 6,853 7,120 7,476 7,832 9,240 9,345 9,790 9,968 10,680 11,748 12,460 13,706 14,685 14,952 15,664 18,480 18,690 19,580 20,559 21,360 23,496 24,920 27,412 29,370 29,904 34,265 37,380 39,160 41,118 46,992 49,840 54,824 58,740 68,530 74,760 78,320 82,236 102,795 109,648 117,480 137,060 149,520 164,472 205,590 234,960 274,120 328,944 411,180 548,240 822,360 and 1,644,720, with no remainder.
Since 1,644,720 cannot be divided by just 1 and 1,644,720, it is not a prime number.