A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 315840 can be evenly divided by 1 2 3 4 5 6 7 8 10 12 14 15 16 20 21 24 28 30 32 35 40 42 47 48 56 60 64 70 80 84 94 96 105 112 120 140 141 160 168 188 192 210 224 235 240 280 282 320 329 336 376 420 448 470 480 560 564 658 672 705 752 840 940 960 987 1,120 1,128 1,316 1,344 1,410 1,504 1,645 1,680 1,880 1,974 2,240 2,256 2,632 2,820 3,008 3,290 3,360 3,760 3,948 4,512 4,935 5,264 5,640 6,580 6,720 7,520 7,896 9,024 9,870 10,528 11,280 13,160 15,040 15,792 19,740 21,056 22,560 26,320 31,584 39,480 45,120 52,640 63,168 78,960 105,280 157,920 and 315,840, with no remainder.
Since 315,840 cannot be divided by just 1 and 315,840, it is not a prime number.