Let's first write all the factors of 168 and 252.
168 - 1,2,3,4,6,7,8,12,14,21,24,28,42,56,84,168
252 - 1,2,3,4,6,7,9,12,14,18,21,28,36,42,63,84,126,252
84 is the GCF because 84 is greatest common factor of 168 and 252.
So, 84 is the GCF
Hope this helps.. :)