Philip P. answered 11/13/17
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49a2x + 7a2y − 21bx − 3by
Factor by grouping:
(49a2x + 7a2y) − (21bx + 3by)
7a2(7x+y) - 3b(7x+y)
Factor out the (7x+y):
(7x+y)(7a2-3b)