Well, look at the equation you're given:

f(n) = (10/9)(12-n)

Think about what this function does. After 1 month, we can find how many households use the product:

f(n) = (10/9)(12 - 1) = 12.2 thousand households

Now, if we KNOW that 3 thousand households use the product, we simply work backwards to find how many months that's at. What part of the function does 3 thousand correspond to?