A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 786600 can be evenly divided by 1 2 3 4 5 6 8 9 10 12 15 18 19 20 23 24 25 30 36 38 40 45 46 50 57 60 69 72 75 76 90 92 95 100 114 115 120 138 150 152 171 180 184 190 200 207 225 228 230 276 285 300 342 345 360 380 414 437 450 456 460 475 552 570 575 600 684 690 760 828 855 874 900 920 950 1,035 1,140 1,150 1,311 1,368 1,380 1,425 1,656 1,710 1,725 1,748 1,800 1,900 2,070 2,185 2,280 2,300 2,622 2,760 2,850 3,420 3,450 3,496 3,800 3,933 4,140 4,275 4,370 4,600 5,175 5,244 5,700 6,555 6,840 6,900 7,866 8,280 8,550 8,740 10,350 10,488 10,925 11,400 13,110 13,800 15,732 17,100 17,480 19,665 20,700 21,850 26,220 31,464 32,775 34,200 39,330 41,400 43,700 52,440 65,550 78,660 87,400 98,325 131,100 157,320 196,650 262,200 393,300 and 786,600, with no remainder.
Since 786,600 cannot be divided by just 1 and 786,600, it is not a prime number.