A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 991800 can be evenly divided by 1 2 3 4 5 6 8 9 10 12 15 18 19 20 24 25 29 30 36 38 40 45 50 57 58 60 72 75 76 87 90 95 100 114 116 120 145 150 152 171 174 180 190 200 225 228 232 261 285 290 300 342 348 360 380 435 450 456 475 522 551 570 580 600 684 696 725 760 855 870 900 950 1,044 1,102 1,140 1,160 1,305 1,368 1,425 1,450 1,653 1,710 1,740 1,800 1,900 2,088 2,175 2,204 2,280 2,610 2,755 2,850 2,900 3,306 3,420 3,480 3,800 4,275 4,350 4,408 4,959 5,220 5,510 5,700 5,800 6,525 6,612 6,840 8,265 8,550 8,700 9,918 10,440 11,020 11,400 13,050 13,224 13,775 16,530 17,100 17,400 19,836 22,040 24,795 26,100 27,550 33,060 34,200 39,672 41,325 49,590 52,200 55,100 66,120 82,650 99,180 110,200 123,975 165,300 198,360 247,950 330,600 495,900 and 991,800, with no remainder.
Since 991,800 cannot be divided by just 1 and 991,800, it is not a prime number.