The Ratio Of Father's Age - Puzzle

The ratio of father's age to his son's age is 7:3.

The product of their age is 756.

The ratio of their ages after 6 years will be:

1) 11:7
2) 13:9
3) 2:1
4) 5:2

Before you go for the answer please comment your answer!

Answer:

3) 2:1