As we can see, the number 3999991 can be expressed as a product of two numbers that are not 1 and the number 3999991 itself, therefore that number is not prime.
Since 21 is divided by 3 you might think that it is equivalent but remeber you have to divide by both sides, so if you divide 21 by 7 it works but 28 by seven does not! Hope this helps!
