Puzzle: Find out the number "?"

If you are an Accountant, maybe 3 months;
If you are a Lawyer, maybe you can never find the answer.

If you are Engineer hope within 2 min.

---+----+----+------+----

2   3   4   15    12

---+----+----+------+----

3   4    5   28   20

---+----+----+------+----

4   5    6  45    30

---+----+----+------+----

5   6    7   66   42

---+----+----+------+----

6   7    8   ?    56

---+----+----+------+----

Find out the number in "?"

Stay Tune
-Sandhya