2mn - 3m + 8n - 12
Let's group the first two terms together and the last two:
2mn - 3m and 8n - 12
In the first group, m is the greatest common factor.
m(2n - 3)
In the second group, 4 is the greatest common factor.
4(2n - 3)
Therefore, (m + 4) and (2n - 3) are our factors. Our missing term is 2n.