A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 997920 can be evenly divided by 1 2 3 4 5 6 7 8 9 10 11 12 14 15 16 18 20 21 22 24 27 28 30 32 33 35 36 40 42 44 45 48 54 55 56 60 63 66 70 72 77 80 81 84 88 90 96 99 105 108 110 112 120 126 132 135 140 144 154 160 162 165 168 176 180 189 198 210 216 220 224 231 240 252 264 270 280 288 297 308 315 324 330 336 352 360 378 385 396 405 420 432 440 462 480 495 504 528 540 560 567 594 616 630 648 660 672 693 720 756 770 792 810 840 864 880 891 924 945 990 1,008 1,056 1,080 1,120 1,134 1,155 1,188 1,232 1,260 1,296 1,320 1,386 1,440 1,485 1,512 1,540 1,584 1,620 1,680 1,760 1,782 1,848 1,890 1,980 2,016 2,079 2,160 2,268 2,310 2,376 2,464 2,520 2,592 2,640 2,772 2,835 2,970 3,024 3,080 3,168 3,240 3,360 3,465 3,564 3,696 3,780 3,960 4,158 4,320 4,455 4,536 4,620 4,752 5,040 5,280 5,544 5,670 5,940 6,048 6,160 6,237 6,480 6,930 7,128 7,392 7,560 7,920 8,316 8,910 9,072 9,240 9,504 10,080 10,395 11,088 11,340 11,880 12,320 12,474 12,960 13,860 14,256 15,120 15,840 16,632 17,820 18,144 18,480 20,790 22,176 22,680 23,760 24,948 27,720 28,512 30,240 31,185 33,264 35,640 36,960 41,580 45,360 47,520 49,896 55,440 62,370 66,528 71,280 83,160 90,720 99,792 110,880 124,740 142,560 166,320 199,584 249,480 332,640 498,960 and 997,920, with no remainder.
Since 997,920 cannot be divided by just 1 and 997,920, it is not a prime number.