Answer:
To find the GCF, do the prime factorization of both numbers. 
 
42 = 2 x 3 x 7 
 
28 = 2 x 2 x 7 
 
 
 
The GCF uses the numbers that appear in BOTH, 2 x 7 = 14 is the GCF of the numbers. There is no variable GCF because there isn't a variable that appears in both. 
 
 
 
Taking out the GCF gives you: 
 
14(3x + 2y)
Explanation: