activity A has a finish - to-finish relationship with activity B. Activity A has a duration of 10 days. activity B has a duration of 9 days with a 3 day lag. what is the total duration of activty a and B?

1.7 days
2.8 days
3.12 days
4. 9 days

please help me with the above question. i did not get how to arrive the answer. hoping someone will reply