Factors:
15 = 1, 3, 5, 15
21 = 1, 3, 7, 21
The GCF is 3
That means that you can take three out of 15 and 21
15/3 = 5
21/3 = 7
so...
3 (5 + 7)
Hope this helped!
~Just a girl in love with Shawn Mendes
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