A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 6126516 can be evenly divided by 1 2 3 4 6 9 11 12 18 22 27 33 36 44 54 66 81 99 108 132 162 191 198 243 297 324 382 396 486 573 594 729 764 891 972 1,146 1,188 1,458 1,719 1,782 2,101 2,292 2,673 2,916 3,438 3,564 4,202 5,157 5,346 6,303 6,876 8,019 8,404 10,314 10,692 12,606 15,471 16,038 18,909 20,628 25,212 30,942 32,076 37,818 46,413 56,727 61,884 75,636 92,826 113,454 139,239 170,181 185,652 226,908 278,478 340,362 510,543 556,956 680,724 1,021,086 1,531,629 2,042,172 3,063,258 and 6,126,516, with no remainder.
Since 6,126,516 cannot be divided by just 1 and 6,126,516, it is not a prime number.