A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 3786720 can be evenly divided by 1 2 3 4 5 6 7 8 10 12 14 15 16 20 21 23 24 28 30 32 35 40 42 46 48 49 56 60 69 70 80 84 92 96 98 105 112 115 120 138 140 147 160 161 168 184 196 210 224 230 240 245 276 280 294 322 336 343 345 368 392 420 460 480 483 490 552 560 588 644 672 686 690 735 736 784 805 840 920 966 980 1,029 1,104 1,120 1,127 1,176 1,288 1,372 1,380 1,470 1,568 1,610 1,680 1,715 1,840 1,932 1,960 2,058 2,208 2,254 2,352 2,415 2,576 2,744 2,760 2,940 3,220 3,360 3,381 3,430 3,680 3,864 3,920 4,116 4,508 4,704 4,830 5,145 5,152 5,488 5,520 5,635 5,880 6,440 6,762 6,860 7,728 7,840 7,889 8,232 9,016 9,660 10,290 10,976 11,040 11,270 11,760 12,880 13,524 13,720 15,456 15,778 16,464 16,905 18,032 19,320 20,580 22,540 23,520 23,667 25,760 27,048 27,440 31,556 32,928 33,810 36,064 38,640 39,445 41,160 45,080 47,334 54,096 54,880 63,112 67,620 77,280 78,890 82,320 90,160 94,668 108,192 118,335 126,224 135,240 157,780 164,640 180,320 189,336 236,670 252,448 270,480 315,560 378,672 473,340 540,960 631,120 757,344 946,680 1,262,240 1,893,360 and 3,786,720, with no remainder.
Since 3,786,720 cannot be divided by just 1 and 3,786,720, it is not a prime number.