its not a addition problem where the answer is 12 + 12 = 24 but rather u factor the equation so lets do that please follow along.
Find the GCF (Greatest common factor)
Factor out the GCF. So basically write the GCF first, then in parentheses, divide each term by the GCF.
Now lastly simplify each term in parenthesis and the answer is 12( m + n )