A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 6601644 can be evenly divided by 1 2 3 4 6 7 9 12 14 17 18 21 23 28 34 36 42 46 51 63 67 68 69 84 92 102 119 126 134 138 153 161 201 204 207 238 252 268 276 306 322 357 391 402 414 469 476 483 603 612 644 714 782 804 828 938 966 1,071 1,139 1,173 1,206 1,407 1,428 1,449 1,541 1,564 1,876 1,932 2,142 2,278 2,346 2,412 2,737 2,814 2,898 3,082 3,417 3,519 4,221 4,284 4,556 4,623 4,692 5,474 5,628 5,796 6,164 6,834 7,038 7,973 8,211 8,442 9,246 10,251 10,787 10,948 13,668 13,869 14,076 15,946 16,422 16,884 18,492 20,502 21,574 23,919 24,633 26,197 27,738 31,892 32,361 32,844 41,004 43,148 47,838 49,266 52,394 55,476 64,722 71,757 78,591 95,676 97,083 98,532 104,788 129,444 143,514 157,182 183,379 194,166 235,773 287,028 314,364 366,758 388,332 471,546 550,137 733,516 943,092 1,100,274 1,650,411 2,200,548 3,300,822 and 6,601,644, with no remainder.
Since 6,601,644 cannot be divided by just 1 and 6,601,644, it is not a prime number.