M is 12 years old and three times of N, what is the age of M when M is two times of N?

First you need to solve for the age of N, which is pretty simple. Simply take the age of M and divide by 3. Now take the age of N times 2.
To get the age of M at the age of N times 2 start with age of M at the beginning and add the age of N at the beginning to get your answer.