GCF stands for "greatest common factor". This means what is the largest quantity you can divide into 6x and 20xyz without a remainder. First see what is the biggest number you can divide 6 by and also 20 without having a remainder. Next, you should see which variables 6x and 20xyz have in common. variables are the (x,y, and z). Put these two values togther and this will be your GCF.
