maybe think of it this way:
You have a numbert to start with, the $\$1170$. Now decide if you are looking for the value before, or after the discounting / increase event.
Let's deal with the after case:
I need the value after increase or decrease, so I need to add or subtract proportion of what I have, to what I have, to get the value after: $$\mathrm{value \;after}=1170\cdot(1+0.12)=1310.4\;\;[increase\;case]$$
$$\mathrm{value \;after}=1170\cdot(1-0.12)=1029.6\;\;[discount\;case]$$
The second line is the calc needed for your question, because you were looking for the value after a discount.
Now let's deal with the before case (here the price change has happened, and I want to reverse its effect to find out where I started:
$$\mathrm{value \;before}=\frac{1310.4}{1+0.12}=1170\;\;[increase\;case]$$
$$\mathrm{value \;before}=\frac{1029.6}{1-0.12}=1170\;\;[discount\;case]$$
Hope that helps?