A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 763980 can be evenly divided by 1 2 3 4 5 6 7 10 12 14 15 17 20 21 28 30 34 35 42 51 60 68 70 84 85 102 105 107 119 140 170 204 210 214 238 255 321 340 357 420 428 476 510 535 595 642 714 749 1,020 1,070 1,190 1,284 1,428 1,498 1,605 1,785 1,819 2,140 2,247 2,380 2,996 3,210 3,570 3,638 3,745 4,494 5,457 6,420 7,140 7,276 7,490 8,988 9,095 10,914 11,235 12,733 14,980 18,190 21,828 22,470 25,466 27,285 36,380 38,199 44,940 50,932 54,570 63,665 76,398 109,140 127,330 152,796 190,995 254,660 381,990 and 763,980, with no remainder.
Since 763,980 cannot be divided by just 1 and 763,980, it is not a prime number.