A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 2166120 can be evenly divided by 1 2 3 4 5 6 8 9 10 11 12 15 18 20 22 24 30 33 36 40 44 45 55 60 66 72 88 90 99 110 120 132 165 180 198 220 264 330 360 396 440 495 547 660 792 990 1,094 1,320 1,641 1,980 2,188 2,735 3,282 3,960 4,376 4,923 5,470 6,017 6,564 8,205 9,846 10,940 12,034 13,128 16,410 18,051 19,692 21,880 24,068 24,615 30,085 32,820 36,102 39,384 48,136 49,230 54,153 60,170 65,640 72,204 90,255 98,460 108,306 120,340 144,408 180,510 196,920 216,612 240,680 270,765 361,020 433,224 541,530 722,040 1,083,060 and 2,166,120, with no remainder.
Since 2,166,120 cannot be divided by just 1 and 2,166,120, it is not a prime number.