A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 915040 can be evenly divided by 1 2 4 5 7 8 10 14 16 19 20 28 32 35 38 40 43 56 70 76 80 86 95 112 133 140 152 160 172 190 215 224 266 280 301 304 344 380 430 532 560 602 608 665 688 760 817 860 1,064 1,120 1,204 1,330 1,376 1,505 1,520 1,634 1,720 2,128 2,408 2,660 3,010 3,040 3,268 3,440 4,085 4,256 4,816 5,320 5,719 6,020 6,536 6,880 8,170 9,632 10,640 11,438 12,040 13,072 16,340 21,280 22,876 24,080 26,144 28,595 32,680 45,752 48,160 57,190 65,360 91,504 114,380 130,720 183,008 228,760 457,520 and 915,040, with no remainder.
Since 915,040 cannot be divided by just 1 and 915,040, it is not a prime number.