A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 7515816 can be evenly divided by 1 2 3 4 6 7 8 11 12 14 21 22 24 28 33 42 44 49 56 66 77 83 84 88 98 132 147 154 166 168 196 231 249 264 294 308 332 343 392 462 498 539 581 588 616 664 686 913 924 996 1,029 1,078 1,162 1,176 1,372 1,617 1,743 1,826 1,848 1,992 2,058 2,156 2,324 2,739 2,744 3,234 3,486 3,652 3,773 4,067 4,116 4,312 4,648 5,478 6,391 6,468 6,972 7,304 7,546 8,134 8,232 10,956 11,319 12,201 12,782 12,936 13,944 15,092 16,268 19,173 21,912 22,638 24,402 25,564 28,469 30,184 32,536 38,346 44,737 45,276 48,804 51,128 56,938 76,692 85,407 89,474 90,552 97,608 113,876 134,211 153,384 170,814 178,948 227,752 268,422 313,159 341,628 357,896 536,844 626,318 683,256 939,477 1,073,688 1,252,636 1,878,954 2,505,272 3,757,908 and 7,515,816, with no remainder.
Since 7,515,816 cannot be divided by just 1 and 7,515,816, it is not a prime number.