A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1275960 can be evenly divided by 1 2 3 4 5 6 7 8 10 12 14 15 20 21 24 28 30 31 35 40 42 49 56 60 62 70 84 93 98 105 120 124 140 147 155 168 186 196 210 217 245 248 280 294 310 343 372 392 420 434 465 490 588 620 651 686 735 744 840 868 930 980 1,029 1,085 1,176 1,240 1,302 1,372 1,470 1,519 1,715 1,736 1,860 1,960 2,058 2,170 2,604 2,744 2,940 3,038 3,255 3,430 3,720 4,116 4,340 4,557 5,145 5,208 5,880 6,076 6,510 6,860 7,595 8,232 8,680 9,114 10,290 10,633 12,152 13,020 13,720 15,190 18,228 20,580 21,266 22,785 26,040 30,380 31,899 36,456 41,160 42,532 45,570 53,165 60,760 63,798 85,064 91,140 106,330 127,596 159,495 182,280 212,660 255,192 318,990 425,320 637,980 and 1,275,960, with no remainder.
Since 1,275,960 cannot be divided by just 1 and 1,275,960, it is not a prime number.