In the given problem it is already given that the seedling grew by 1/3 inch in the first week and 5/6 inch in the second week. Then it would not be a problem to find the total length of growth of the seedling.
The length up to which the seedling grew in 2 weeks = (1/3) + (5/6)
= (2 + 5)/6
= 7/6 inch
So the seedling grew by 7/6 inches in two weeks. I hope that this answer is what you were looking for. You can solve such problems in the future by following the method that is applied for solving this problem.