A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 309120 can be evenly divided by 1 2 3 4 5 6 7 8 10 12 14 15 16 20 21 23 24 28 30 32 35 40 42 46 48 56 60 64 69 70 80 84 92 96 105 112 115 120 128 138 140 160 161 168 184 192 210 224 230 240 276 280 320 322 336 345 368 384 420 448 460 480 483 552 560 640 644 672 690 736 805 840 896 920 960 966 1,104 1,120 1,288 1,344 1,380 1,472 1,610 1,680 1,840 1,920 1,932 2,208 2,240 2,415 2,576 2,688 2,760 2,944 3,220 3,360 3,680 3,864 4,416 4,480 4,830 5,152 5,520 6,440 6,720 7,360 7,728 8,832 9,660 10,304 11,040 12,880 13,440 14,720 15,456 19,320 20,608 22,080 25,760 30,912 38,640 44,160 51,520 61,824 77,280 103,040 154,560 and 309,120, with no remainder.
Since 309,120 cannot be divided by just 1 and 309,120, it is not a prime number.