A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 5918640 can be evenly divided by 1 2 3 4 5 6 7 8 10 12 13 14 15 16 20 21 24 26 28 30 35 39 40 42 48 52 56 60 65 70 78 80 84 91 104 105 112 120 130 140 156 168 182 195 208 210 240 260 271 273 280 312 336 364 390 420 455 520 542 546 560 624 728 780 813 840 910 1,040 1,084 1,092 1,355 1,365 1,456 1,560 1,626 1,680 1,820 1,897 2,168 2,184 2,710 2,730 3,120 3,252 3,523 3,640 3,794 4,065 4,336 4,368 5,420 5,460 5,691 6,504 7,046 7,280 7,588 8,130 9,485 10,569 10,840 10,920 11,382 13,008 14,092 15,176 16,260 17,615 18,970 21,138 21,680 21,840 22,764 24,661 28,184 28,455 30,352 32,520 35,230 37,940 42,276 45,528 49,322 52,845 56,368 56,910 65,040 70,460 73,983 75,880 84,552 91,056 98,644 105,690 113,820 123,305 140,920 147,966 151,760 169,104 197,288 211,380 227,640 246,610 281,840 295,932 369,915 394,576 422,760 455,280 493,220 591,864 739,830 845,520 986,440 1,183,728 1,479,660 1,972,880 2,959,320 and 5,918,640, with no remainder.
Since 5,918,640 cannot be divided by just 1 and 5,918,640, it is not a prime number.