Question 294049
We need to factor.

Make two groups and factor individually.

3m^2 + 3mn = Group A

m + n = Group B

NOTICE: There is nothing that can be done about Group B.  It is already in factored form.

3m^2 + 3mn becomes, after factoring, 3m(m + n).

We now have 3m(m + n) + 1 (m + n).

Do you see the quantity (m + n)?  We only need one of them as a factor.

ANSWER: (3m + 1)(m + n)

NOTE: I used the Grouping Method of factoring to find my answer.