If the company constructs 9 apartment buildings, the number of stores will be
3
4
5
6
if the company is producing at point c, the opportunity cost of building 3 more stores will be buildings.
3
4
5
6

1. I believe the answer is 3 

You just need to see and compare it with the given picture

The Vertical Line  |   represent number of Stores.
The horizontal line _ represent number of Apartments.
Take a look at point E. 
When the apartment is 9, the stores that they could build is 3.

2. I believe the answer is 6.
In point C, the company produces 6 Apartments and 7 Stores. If the company want to build 3 more stores it would become 10 stores.
Take a look at point A. Where the stores are 10, the company cannot build any appartment, which means that they had to sacrifice all 6 from point C.