A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 9721620 can be evenly divided by 1 2 3 4 5 6 9 10 12 15 17 18 20 27 30 34 36 45 51 54 60 68 81 85 90 102 108 135 153 162 170 180 204 255 270 306 324 340 353 405 459 510 540 612 706 765 810 918 1,020 1,059 1,377 1,412 1,530 1,620 1,765 1,836 2,118 2,295 2,754 3,060 3,177 3,530 4,236 4,590 5,295 5,508 6,001 6,354 6,885 7,060 9,180 9,531 10,590 12,002 12,708 13,770 15,885 18,003 19,062 21,180 24,004 27,540 28,593 30,005 31,770 36,006 38,124 47,655 54,009 57,186 60,010 63,540 72,012 90,015 95,310 108,018 114,372 120,020 142,965 162,027 180,030 190,620 216,036 270,045 285,930 324,054 360,060 486,081 540,090 571,860 648,108 810,135 972,162 1,080,180 1,620,270 1,944,324 2,430,405 3,240,540 4,860,810 and 9,721,620, with no remainder.
Since 9,721,620 cannot be divided by just 1 and 9,721,620, it is not a prime number.