A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 6660864 can be evenly divided by 1 2 3 4 6 7 8 9 12 14 16 18 21 24 28 32 36 42 48 49 56 59 63 64 72 84 96 98 112 118 126 128 144 147 168 177 192 196 224 236 252 256 288 294 336 354 384 392 413 441 448 472 504 531 576 588 672 708 768 784 826 882 896 944 1,008 1,062 1,152 1,176 1,239 1,344 1,416 1,568 1,652 1,764 1,792 1,888 2,016 2,124 2,304 2,352 2,478 2,688 2,832 2,891 3,136 3,304 3,528 3,717 3,776 4,032 4,248 4,704 4,956 5,376 5,664 5,782 6,272 6,608 7,056 7,434 7,552 8,064 8,496 8,673 9,408 9,912 11,328 11,564 12,544 13,216 14,112 14,868 15,104 16,128 16,992 17,346 18,816 19,824 22,656 23,128 26,019 26,432 28,224 29,736 33,984 34,692 37,632 39,648 45,312 46,256 52,038 52,864 56,448 59,472 67,968 69,384 79,296 92,512 104,076 105,728 112,896 118,944 135,936 138,768 158,592 185,024 208,152 237,888 277,536 317,184 370,048 416,304 475,776 555,072 740,096 832,608 951,552 1,110,144 1,665,216 2,220,288 3,330,432 and 6,660,864, with no remainder.
Since 6,660,864 cannot be divided by just 1 and 6,660,864, it is not a prime number.