A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 3991050 can be evenly divided by 1 2 3 5 6 7 9 10 14 15 18 21 25 30 35 42 45 49 50 63 70 75 90 98 105 126 147 150 175 181 210 225 245 294 315 350 362 441 450 490 525 543 630 735 882 905 1,050 1,086 1,225 1,267 1,470 1,575 1,629 1,810 2,205 2,450 2,534 2,715 3,150 3,258 3,675 3,801 4,410 4,525 5,430 6,335 7,350 7,602 8,145 8,869 9,050 11,025 11,403 12,670 13,575 16,290 17,738 19,005 22,050 22,806 26,607 27,150 31,675 38,010 40,725 44,345 53,214 57,015 63,350 79,821 81,450 88,690 95,025 114,030 133,035 159,642 190,050 221,725 266,070 285,075 399,105 443,450 570,150 665,175 798,210 1,330,350 1,995,525 and 3,991,050, with no remainder.
Since 3,991,050 cannot be divided by just 1 and 3,991,050, it is not a prime number.