A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 661440 can be evenly divided by 1 2 3 4 5 6 8 10 12 13 15 16 20 24 26 30 32 39 40 48 52 53 60 64 65 78 80 96 104 106 120 130 156 159 160 192 195 208 212 240 260 265 312 318 320 390 416 424 480 520 530 624 636 689 780 795 832 848 960 1,040 1,060 1,248 1,272 1,378 1,560 1,590 1,696 2,067 2,080 2,120 2,496 2,544 2,756 3,120 3,180 3,392 3,445 4,134 4,160 4,240 5,088 5,512 6,240 6,360 6,890 8,268 8,480 10,176 10,335 11,024 12,480 12,720 13,780 16,536 16,960 20,670 22,048 25,440 27,560 33,072 41,340 44,096 50,880 55,120 66,144 82,680 110,240 132,288 165,360 220,480 330,720 and 661,440, with no remainder.
Since 661,440 cannot be divided by just 1 and 661,440, it is not a prime number.