A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1611960 can be evenly divided by 1 2 3 4 5 6 7 8 10 12 14 15 19 20 21 24 28 30 35 38 40 42 56 57 60 70 76 84 95 101 105 114 120 133 140 152 168 190 202 210 228 266 280 285 303 380 399 404 420 456 505 532 570 606 665 707 760 798 808 840 1,010 1,064 1,140 1,212 1,330 1,414 1,515 1,596 1,919 1,995 2,020 2,121 2,280 2,424 2,660 2,828 3,030 3,192 3,535 3,838 3,990 4,040 4,242 5,320 5,656 5,757 6,060 7,070 7,676 7,980 8,484 9,595 10,605 11,514 12,120 13,433 14,140 15,352 15,960 16,968 19,190 21,210 23,028 26,866 28,280 28,785 38,380 40,299 42,420 46,056 53,732 57,570 67,165 76,760 80,598 84,840 107,464 115,140 134,330 161,196 201,495 230,280 268,660 322,392 402,990 537,320 805,980 and 1,611,960, with no remainder.
Since 1,611,960 cannot be divided by just 1 and 1,611,960, it is not a prime number.