A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1545840 can be evenly divided by 1 2 3 4 5 6 8 9 10 12 15 16 18 19 20 24 30 36 38 40 45 48 57 60 72 76 80 90 95 113 114 120 144 152 171 180 190 226 228 240 285 304 339 342 360 380 452 456 565 570 678 684 720 760 855 904 912 1,017 1,130 1,140 1,356 1,368 1,520 1,695 1,710 1,808 2,034 2,147 2,260 2,280 2,712 2,736 3,390 3,420 4,068 4,294 4,520 4,560 5,085 5,424 6,441 6,780 6,840 8,136 8,588 9,040 10,170 10,735 12,882 13,560 13,680 16,272 17,176 19,323 20,340 21,470 25,764 27,120 32,205 34,352 38,646 40,680 42,940 51,528 64,410 77,292 81,360 85,880 96,615 103,056 128,820 154,584 171,760 193,230 257,640 309,168 386,460 515,280 772,920 and 1,545,840, with no remainder.
Since 1,545,840 cannot be divided by just 1 and 1,545,840, it is not a prime number.