A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 644670 can be evenly divided by 1 2 3 5 6 9 10 13 15 18 19 26 29 30 38 39 45 57 58 65 78 87 90 95 114 117 130 145 171 174 190 195 234 247 261 285 290 342 377 390 435 494 522 551 570 585 741 754 855 870 1,102 1,131 1,170 1,235 1,305 1,482 1,653 1,710 1,885 2,223 2,262 2,470 2,610 2,755 3,306 3,393 3,705 3,770 4,446 4,959 5,510 5,655 6,786 7,163 7,410 8,265 9,918 11,115 11,310 14,326 16,530 16,965 21,489 22,230 24,795 33,930 35,815 42,978 49,590 64,467 71,630 107,445 128,934 214,890 322,335 and 644,670, with no remainder.
Since 644,670 cannot be divided by just 1 and 644,670, it is not a prime number.