A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 2655120 can be evenly divided by 1 2 3 4 5 6 8 10 12 13 15 16 20 23 24 26 30 37 39 40 46 48 52 60 65 69 74 78 80 92 104 111 115 120 130 138 148 156 184 185 195 208 222 230 240 260 276 296 299 312 345 368 370 390 444 460 481 520 552 555 592 598 624 690 740 780 851 888 897 920 962 1,040 1,104 1,110 1,196 1,380 1,443 1,480 1,495 1,560 1,702 1,776 1,794 1,840 1,924 2,220 2,392 2,405 2,553 2,760 2,886 2,960 2,990 3,120 3,404 3,588 3,848 4,255 4,440 4,485 4,784 4,810 5,106 5,520 5,772 5,980 6,808 7,176 7,215 7,696 8,510 8,880 8,970 9,620 10,212 11,063 11,544 11,960 12,765 13,616 14,352 14,430 17,020 17,940 19,240 20,424 22,126 23,088 23,920 25,530 28,860 33,189 34,040 35,880 38,480 40,848 44,252 51,060 55,315 57,720 66,378 68,080 71,760 88,504 102,120 110,630 115,440 132,756 165,945 177,008 204,240 221,260 265,512 331,890 442,520 531,024 663,780 885,040 1,327,560 and 2,655,120, with no remainder.
Since 2,655,120 cannot be divided by just 1 and 2,655,120, it is not a prime number.