A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 789750 can be evenly divided by 1 2 3 5 6 9 10 13 15 18 25 26 27 30 39 45 50 54 65 75 78 81 90 117 125 130 135 150 162 195 225 234 243 250 270 325 351 375 390 405 450 486 585 650 675 702 750 810 975 1,053 1,125 1,170 1,215 1,350 1,625 1,755 1,950 2,025 2,106 2,250 2,430 2,925 3,159 3,250 3,375 3,510 4,050 4,875 5,265 5,850 6,075 6,318 6,750 8,775 9,750 10,125 10,530 12,150 14,625 15,795 17,550 20,250 26,325 29,250 30,375 31,590 43,875 52,650 60,750 78,975 87,750 131,625 157,950 263,250 394,875 and 789,750, with no remainder.
Since 789,750 cannot be divided by just 1 and 789,750, it is not a prime number.