A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 871920 can be evenly divided by 1 2 3 4 5 6 7 8 9 10 12 14 15 16 18 20 21 24 28 30 35 36 40 42 45 48 56 60 63 70 72 80 84 90 105 112 120 126 140 144 168 173 180 210 240 252 280 315 336 346 360 420 504 519 560 630 692 720 840 865 1,008 1,038 1,211 1,260 1,384 1,557 1,680 1,730 2,076 2,422 2,520 2,595 2,768 3,114 3,460 3,633 4,152 4,844 5,040 5,190 6,055 6,228 6,920 7,266 7,785 8,304 9,688 10,380 10,899 12,110 12,456 13,840 14,532 15,570 18,165 19,376 20,760 21,798 24,220 24,912 29,064 31,140 36,330 41,520 43,596 48,440 54,495 58,128 62,280 72,660 87,192 96,880 108,990 124,560 145,320 174,384 217,980 290,640 435,960 and 871,920, with no remainder.
Since 871,920 cannot be divided by just 1 and 871,920, it is not a prime number.