A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 3401874 can be evenly divided by 1 2 3 6 7 9 14 18 19 21 29 38 42 49 57 58 63 87 98 114 126 133 147 171 174 203 261 266 294 342 343 399 406 441 522 551 609 686 798 882 931 1,029 1,102 1,197 1,218 1,421 1,653 1,827 1,862 2,058 2,394 2,793 2,842 3,087 3,306 3,654 3,857 4,263 4,959 5,586 6,174 6,517 7,714 8,379 8,526 9,918 9,947 11,571 12,789 13,034 16,758 19,551 19,894 23,142 25,578 26,999 29,841 34,713 39,102 53,998 58,653 59,682 69,426 80,997 89,523 117,306 161,994 179,046 188,993 242,991 377,986 485,982 566,979 1,133,958 1,700,937 and 3,401,874, with no remainder.
Since 3,401,874 cannot be divided by just 1 and 3,401,874, it is not a prime number.