A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 7800975 can be evenly divided by 1 3 5 7 9 13 15 21 25 27 35 39 45 63 65 75 91 105 117 127 135 175 189 195 225 273 315 325 351 381 455 525 585 635 675 819 889 945 975 1,143 1,365 1,575 1,651 1,755 1,905 2,275 2,457 2,667 2,925 3,175 3,429 4,095 4,445 4,725 4,953 5,715 6,825 8,001 8,255 8,775 9,525 11,557 12,285 13,335 14,859 17,145 20,475 22,225 24,003 24,765 28,575 34,671 40,005 41,275 44,577 57,785 61,425 66,675 74,295 85,725 104,013 120,015 123,825 173,355 200,025 222,885 288,925 312,039 371,475 520,065 600,075 866,775 1,114,425 1,560,195 2,600,325 and 7,800,975, with no remainder.
Since 7,800,975 cannot be divided by just 1 and 7,800,975, it is not a prime number.