A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 3405168 can be evenly divided by 1 2 3 4 6 8 9 12 13 16 17 18 24 26 34 36 39 48 51 52 68 72 78 102 104 107 117 136 144 153 156 204 208 214 221 234 272 306 312 321 408 428 442 468 612 624 642 663 816 856 884 936 963 1,224 1,284 1,326 1,391 1,712 1,768 1,819 1,872 1,926 1,989 2,448 2,568 2,652 2,782 3,536 3,638 3,852 3,978 4,173 5,136 5,304 5,457 5,564 7,276 7,704 7,956 8,346 10,608 10,914 11,128 12,519 14,552 15,408 15,912 16,371 16,692 21,828 22,256 23,647 25,038 29,104 31,824 32,742 33,384 43,656 47,294 50,076 65,484 66,768 70,941 87,312 94,588 100,152 130,968 141,882 189,176 200,304 212,823 261,936 283,764 378,352 425,646 567,528 851,292 1,135,056 1,702,584 and 3,405,168, with no remainder.
Since 3,405,168 cannot be divided by just 1 and 3,405,168, it is not a prime number.