A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 3166920 can be evenly divided by 1 2 3 4 5 6 8 9 10 12 15 18 19 20 24 30 36 38 40 45 57 60 72 76 90 95 114 120 152 171 180 190 228 285 342 360 380 456 463 570 684 760 855 926 1,140 1,368 1,389 1,710 1,852 2,280 2,315 2,778 3,420 3,704 4,167 4,630 5,556 6,840 6,945 8,334 8,797 9,260 11,112 13,890 16,668 17,594 18,520 20,835 26,391 27,780 33,336 35,188 41,670 43,985 52,782 55,560 70,376 79,173 83,340 87,970 105,564 131,955 158,346 166,680 175,940 211,128 263,910 316,692 351,880 395,865 527,820 633,384 791,730 1,055,640 1,583,460 and 3,166,920, with no remainder.
Since 3,166,920 cannot be divided by just 1 and 3,166,920, it is not a prime number.