A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 7787178 can be evenly divided by 1 2 3 6 7 9 14 18 21 27 42 49 54 63 81 98 109 126 147 162 189 218 243 294 327 378 441 486 567 654 729 763 882 981 1,134 1,323 1,458 1,526 1,701 1,962 2,289 2,646 2,943 3,402 3,969 4,578 5,103 5,341 5,886 6,867 7,938 8,829 10,206 10,682 11,907 13,734 16,023 17,658 20,601 23,814 26,487 32,046 35,721 41,202 48,069 52,974 61,803 71,442 79,461 96,138 123,606 144,207 158,922 185,409 288,414 370,818 432,621 556,227 865,242 1,112,454 1,297,863 2,595,726 3,893,589 and 7,787,178, with no remainder.
Since 7,787,178 cannot be divided by just 1 and 7,787,178, it is not a prime number.