A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1280448 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 64 72 76 78 81 96 104 108 114 117 144 152 156 162 171 192 208 216 228 234 247 288 304 312 324 342 351 416 432 456 468 494 513 576 608 624 648 684 702 741 832 864 912 936 988 1,026 1,053 1,216 1,248 1,296 1,368 1,404 1,482 1,539 1,728 1,824 1,872 1,976 2,052 2,106 2,223 2,496 2,592 2,736 2,808 2,964 3,078 3,648 3,744 3,952 4,104 4,212 4,446 5,184 5,472 5,616 5,928 6,156 6,669 7,488 7,904 8,208 8,424 8,892 10,944 11,232 11,856 12,312 13,338 15,808 16,416 16,848 17,784 20,007 22,464 23,712 24,624 26,676 32,832 33,696 35,568 40,014 47,424 49,248 53,352 67,392 71,136 80,028 98,496 106,704 142,272 160,056 213,408 320,112 426,816 640,224 and 1,280,448, with no remainder.
Since 1,280,448 cannot be divided by just 1 and 1,280,448, it is not a prime number.