Answer:
Ima say the answer is 9, likely she bought 6 more new and with the fire half were destroyed of this 6 new posters only are 3 left so she has 12 posters left that means only 3 are added to how much posters she had before, so in some words she started with 9 also 9+3=12.
For where it says write and equation I think the answer would be something like 9+6-3. In where it says work I want to assume is your explaination so you can use the first paragraph I made. Check Its prob a re do of the work you have done to make sure its right. Coming with "Define the value" Im not sure of this but i think is to define your equation.