A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 640224 can be evenly divided by 1 2 3 4 6 8 9 12 13 16 18 19 24 26 27 32 36 38 39 48 52 54 57 72 76 78 81 96 104 108 114 117 144 152 156 162 171 208 216 228 234 247 288 304 312 324 342 351 416 432 456 468 494 513 608 624 648 684 702 741 864 912 936 988 1,026 1,053 1,248 1,296 1,368 1,404 1,482 1,539 1,824 1,872 1,976 2,052 2,106 2,223 2,592 2,736 2,808 2,964 3,078 3,744 3,952 4,104 4,212 4,446 5,472 5,616 5,928 6,156 6,669 7,904 8,208 8,424 8,892 11,232 11,856 12,312 13,338 16,416 16,848 17,784 20,007 23,712 24,624 26,676 33,696 35,568 40,014 49,248 53,352 71,136 80,028 106,704 160,056 213,408 320,112 and 640,224, with no remainder.
Since 640,224 cannot be divided by just 1 and 640,224, it is not a prime number.