A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 2646240 can be evenly divided by 1 2 3 4 5 6 8 10 12 15 16 20 24 30 32 37 40 48 60 74 80 96 111 120 148 149 160 185 222 240 296 298 370 444 447 480 555 592 596 740 745 888 894 1,110 1,184 1,192 1,480 1,490 1,776 1,788 2,220 2,235 2,384 2,960 2,980 3,552 3,576 4,440 4,470 4,768 5,513 5,920 5,960 7,152 8,880 8,940 11,026 11,920 14,304 16,539 17,760 17,880 22,052 23,840 27,565 33,078 35,760 44,104 55,130 66,156 71,520 82,695 88,208 110,260 132,312 165,390 176,416 220,520 264,624 330,780 441,040 529,248 661,560 882,080 1,323,120 and 2,646,240, with no remainder.
Since 2,646,240 cannot be divided by just 1 and 2,646,240, it is not a prime number.