A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 915936 can be evenly divided by 1 2 3 4 6 7 8 12 14 16 21 24 28 29 32 42 47 48 56 58 84 87 94 96 112 116 141 168 174 188 203 224 232 282 329 336 348 376 406 464 564 609 658 672 696 752 812 928 987 1,128 1,218 1,316 1,363 1,392 1,504 1,624 1,974 2,256 2,436 2,632 2,726 2,784 3,248 3,948 4,089 4,512 4,872 5,264 5,452 6,496 7,896 8,178 9,541 9,744 10,528 10,904 15,792 16,356 19,082 19,488 21,808 28,623 31,584 32,712 38,164 43,616 57,246 65,424 76,328 114,492 130,848 152,656 228,984 305,312 457,968 and 915,936, with no remainder.
Since 915,936 cannot be divided by just 1 and 915,936, it is not a prime number.