There is no common 'x' variable, but each term has '4' as a common factor, hence '4' can be factored but no 'x'. It's also worth nothing that '4' is factored to 2^2, the prime factors.   The changing of 4 to 2^2 is simply a feature of the software, in that all integers are prime factored. Prime factorization is a good practice because it helps identify common terms in the numerator and denominator of any fraction and therefore ensures that cancellations are properly recognized.