What is the GCF of 12x3y5 + 18x4y2?

That would be 6x3y2. For a GCF, you have to look at what each expression has in common. 12 and 18 have the GCF of 6, so 6 is your starting number. Now you have an x4 and and x3. The greatest amount you can pull from both numbers is an x3. X3 is your second part (should be 6x3 now). Now look at your Y values. Y5 and Y2 can both have Y2 taken out of them. So full answer is 6x3y2