GCF=42.
84=2*42, 210=5*42
One way of solving this type of problem is to break down each number into its factors:
84=2*2*3*7
210=2*3*5*7
Then we write down the factors common to each: 2 3 7 and simply multiply them together: 2*3*7=42.
So 42 is the GCF.