A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 784350 can be evenly divided by 1 2 3 5 6 7 9 10 14 15 18 21 25 27 30 35 42 45 50 54 63 70 75 83 90 105 126 135 150 166 175 189 210 225 249 270 315 350 378 415 450 498 525 581 630 675 747 830 945 1,050 1,162 1,245 1,350 1,494 1,575 1,743 1,890 2,075 2,241 2,490 2,905 3,150 3,486 3,735 4,150 4,482 4,725 5,229 5,810 6,225 7,470 8,715 9,450 10,458 11,205 12,450 14,525 15,687 17,430 18,675 22,410 26,145 29,050 31,374 37,350 43,575 52,290 56,025 78,435 87,150 112,050 130,725 156,870 261,450 392,175 and 784,350, with no remainder.
Since 784,350 cannot be divided by just 1 and 784,350, it is not a prime number.