A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 661248 can be evenly divided by 1 2 3 4 6 7 8 9 12 14 16 18 21 24 28 32 36 41 42 48 56 63 64 72 82 84 96 112 123 126 128 144 164 168 192 224 246 252 256 287 288 328 336 369 384 448 492 504 574 576 656 672 738 768 861 896 984 1,008 1,148 1,152 1,312 1,344 1,476 1,722 1,792 1,968 2,016 2,296 2,304 2,583 2,624 2,688 2,952 3,444 3,936 4,032 4,592 5,166 5,248 5,376 5,904 6,888 7,872 8,064 9,184 10,332 10,496 11,808 13,776 15,744 16,128 18,368 20,664 23,616 27,552 31,488 36,736 41,328 47,232 55,104 73,472 82,656 94,464 110,208 165,312 220,416 330,624 and 661,248, with no remainder.
Since 661,248 cannot be divided by just 1 and 661,248, it is not a prime number.