A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1643424 can be evenly divided by 1 2 3 4 6 8 12 16 17 19 24 32 34 38 48 51 53 57 68 76 96 102 106 114 136 152 159 204 212 228 272 304 318 323 408 424 456 544 608 636 646 816 848 901 912 969 1,007 1,272 1,292 1,632 1,696 1,802 1,824 1,938 2,014 2,544 2,584 2,703 3,021 3,604 3,876 4,028 5,088 5,168 5,406 6,042 7,208 7,752 8,056 10,336 10,812 12,084 14,416 15,504 16,112 17,119 21,624 24,168 28,832 31,008 32,224 34,238 43,248 48,336 51,357 68,476 86,496 96,672 102,714 136,952 205,428 273,904 410,856 547,808 821,712 and 1,643,424, with no remainder.
Since 1,643,424 cannot be divided by just 1 and 1,643,424, it is not a prime number.