3m+mn+21+7n

Group the first two terms and the last two terms.

(3m+mn)+21+7n

Factor out the greatest common factor (GCF) from each group.

m⋅(3+n)+7(3+n)

m⋅(3+n)+7(3+n)

Factor the polynomial by factoring out the greatest common factor, 3+n.

(3+n)(m+7)

Factor by Grouping 3m+mn+21+7n