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happyhelper
The greatest common factor is 21, because it can go into 63 three times, and can go into 84 four times, while no other number is higher that can go into both.
ollieboyne
To find the greatest common factor, first split the numbers up into their prime factors:

63 = 3 x 3 x 7
84 = 2 x 2 x 3 x 7

Now multiply all the numbers that appear twice to get the Highest common factor (HCF)

HCF = 3 x 7

= 21

= 188