A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 681600 can be evenly divided by 1 2 3 4 5 6 8 10 12 15 16 20 24 25 30 32 40 48 50 60 64 71 75 80 96 100 120 128 142 150 160 192 200 213 240 284 300 320 355 384 400 426 480 568 600 640 710 800 852 960 1,065 1,136 1,200 1,420 1,600 1,704 1,775 1,920 2,130 2,272 2,400 2,840 3,200 3,408 3,550 4,260 4,544 4,800 5,325 5,680 6,816 7,100 8,520 9,088 9,600 10,650 11,360 13,632 14,200 17,040 21,300 22,720 27,264 28,400 34,080 42,600 45,440 56,800 68,160 85,200 113,600 136,320 170,400 227,200 340,800 and 681,600, with no remainder.
Since 681,600 cannot be divided by just 1 and 681,600, it is not a prime number.