A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 231840 can be evenly divided by 1 2 3 4 5 6 7 8 9 10 12 14 15 16 18 20 21 23 24 28 30 32 35 36 40 42 45 46 48 56 60 63 69 70 72 80 84 90 92 96 105 112 115 120 126 138 140 144 160 161 168 180 184 207 210 224 230 240 252 276 280 288 315 322 336 345 360 368 414 420 460 480 483 504 552 560 630 644 672 690 720 736 805 828 840 920 966 1,008 1,035 1,104 1,120 1,260 1,288 1,380 1,440 1,449 1,610 1,656 1,680 1,840 1,932 2,016 2,070 2,208 2,415 2,520 2,576 2,760 2,898 3,220 3,312 3,360 3,680 3,864 4,140 4,830 5,040 5,152 5,520 5,796 6,440 6,624 7,245 7,728 8,280 9,660 10,080 11,040 11,592 12,880 14,490 15,456 16,560 19,320 23,184 25,760 28,980 33,120 38,640 46,368 57,960 77,280 115,920 and 231,840, with no remainder.
Since 231,840 cannot be divided by just 1 and 231,840, it is not a prime number.