any even number can be represented as 2n where n is an integer
so any 2 even numbers can be represented as 2n and 2m
obviously, one of the common factors is 2
if m and n have no more common factors, then 2 is the GCF and the GCF is even
if m and n have a common factor of x, then the GCF is 2x which is even because x is an integer
true
