A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 771750 can be evenly divided by 1 2 3 5 6 7 9 10 14 15 18 21 25 30 35 42 45 49 50 63 70 75 90 98 105 125 126 147 150 175 210 225 245 250 294 315 343 350 375 441 450 490 525 630 686 735 750 875 882 1,029 1,050 1,125 1,225 1,470 1,575 1,715 1,750 2,058 2,205 2,250 2,450 2,625 3,087 3,150 3,430 3,675 4,410 5,145 5,250 6,125 6,174 7,350 7,875 8,575 10,290 11,025 12,250 15,435 15,750 17,150 18,375 22,050 25,725 30,870 36,750 42,875 51,450 55,125 77,175 85,750 110,250 128,625 154,350 257,250 385,875 and 771,750, with no remainder.
Since 771,750 cannot be divided by just 1 and 771,750, it is not a prime number.