A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 311220 can be evenly divided by 1 2 3 4 5 6 7 9 10 12 13 14 15 18 19 20 21 26 28 30 35 36 38 39 42 45 52 57 60 63 65 70 76 78 84 90 91 95 105 114 117 126 130 133 140 156 171 180 182 190 195 210 228 234 247 252 260 266 273 285 315 342 364 380 390 399 420 455 468 494 532 546 570 585 630 665 684 741 780 798 819 855 910 988 1,092 1,140 1,170 1,197 1,235 1,260 1,330 1,365 1,482 1,596 1,638 1,710 1,729 1,820 1,995 2,223 2,340 2,394 2,470 2,660 2,730 2,964 3,276 3,420 3,458 3,705 3,990 4,095 4,446 4,788 4,940 5,187 5,460 5,985 6,916 7,410 7,980 8,190 8,645 8,892 10,374 11,115 11,970 14,820 15,561 16,380 17,290 20,748 22,230 23,940 25,935 31,122 34,580 44,460 51,870 62,244 77,805 103,740 155,610 and 311,220, with no remainder.
Since 311,220 cannot be divided by just 1 and 311,220, it is not a prime number.