A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 3871260 can be evenly divided by 1 2 3 4 5 6 9 10 12 15 18 20 27 30 36 45 54 60 67 90 107 108 134 135 180 201 214 268 270 321 335 402 428 535 540 603 642 670 804 963 1,005 1,070 1,206 1,284 1,340 1,605 1,809 1,926 2,010 2,140 2,412 2,889 3,015 3,210 3,618 3,852 4,020 4,815 5,778 6,030 6,420 7,169 7,236 9,045 9,630 11,556 12,060 14,338 14,445 18,090 19,260 21,507 28,676 28,890 35,845 36,180 43,014 57,780 64,521 71,690 86,028 107,535 129,042 143,380 193,563 215,070 258,084 322,605 387,126 430,140 645,210 774,252 967,815 1,290,420 1,935,630 and 3,871,260, with no remainder.
Since 3,871,260 cannot be divided by just 1 and 3,871,260, it is not a prime number.