TCP congestion problem
This is a home work problem.
By the way the problem is ambiguous.
For concept see       3.6 pg259 of kurose 6th edition.

As for the solution normally what AIMD do is after the time out the it set window size to 1 in the next slow start phase
In sort the window increase in this manner 2,4,8,9,10,1,2,4,5,6
So after 10th transmission window size will be 6
But in question says the slow start phase start with 2 MSS So to consider that the window will Grow this way 
2,4,8(threshold reached additive increase),9,10(time out&amp; set threshold to 5),2,4,5(threshold reached additive increase),6,7