Question

Prove that 3999991 is not prime

Answer 1

$3999991=4000000-9=2,000^2-3^2=(2000-3)(2000+3)=1997\cdot2003$

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.