A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 3261120 can be evenly divided by 1 2 3 4 5 6 8 10 12 15 16 20 24 30 32 40 43 48 60 64 79 80 86 96 120 129 158 160 172 192 215 237 240 258 316 320 344 395 430 474 480 516 632 645 688 790 860 948 960 1,032 1,185 1,264 1,290 1,376 1,580 1,720 1,896 2,064 2,370 2,528 2,580 2,752 3,160 3,397 3,440 3,792 4,128 4,740 5,056 5,160 6,320 6,794 6,880 7,584 8,256 9,480 10,191 10,320 12,640 13,588 13,760 15,168 16,985 18,960 20,382 20,640 25,280 27,176 33,970 37,920 40,764 41,280 50,955 54,352 67,940 75,840 81,528 101,910 108,704 135,880 163,056 203,820 217,408 271,760 326,112 407,640 543,520 652,224 815,280 1,087,040 1,630,560 and 3,261,120, with no remainder.
Since 3,261,120 cannot be divided by just 1 and 3,261,120, it is not a prime number.