A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 726432 can be evenly divided by 1 2 3 4 6 7 8 12 14 16 21 23 24 28 32 42 46 47 48 56 69 84 92 94 96 112 138 141 161 168 184 188 224 276 282 322 329 336 368 376 483 552 564 644 658 672 736 752 966 987 1,081 1,104 1,128 1,288 1,316 1,504 1,932 1,974 2,162 2,208 2,256 2,576 2,632 3,243 3,864 3,948 4,324 4,512 5,152 5,264 6,486 7,567 7,728 7,896 8,648 10,528 12,972 15,134 15,456 15,792 17,296 22,701 25,944 30,268 31,584 34,592 45,402 51,888 60,536 90,804 103,776 121,072 181,608 242,144 363,216 and 726,432, with no remainder.
Since 726,432 cannot be divided by just 1 and 726,432, it is not a prime number.