A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1262240 can be evenly divided by 1 2 4 5 7 8 10 14 16 20 23 28 32 35 40 46 49 56 70 80 92 98 112 115 140 160 161 184 196 224 230 245 280 322 343 368 392 460 490 560 644 686 736 784 805 920 980 1,120 1,127 1,288 1,372 1,568 1,610 1,715 1,840 1,960 2,254 2,576 2,744 3,220 3,430 3,680 3,920 4,508 5,152 5,488 5,635 6,440 6,860 7,840 7,889 9,016 10,976 11,270 12,880 13,720 15,778 18,032 22,540 25,760 27,440 31,556 36,064 39,445 45,080 54,880 63,112 78,890 90,160 126,224 157,780 180,320 252,448 315,560 631,120 and 1,262,240, with no remainder.
Since 1,262,240 cannot be divided by just 1 and 1,262,240, it is not a prime number.