Do I have to multiply

If the book is 25% off, then that means you paid 75% of the original price. (100% - 25% savings)

This means $12 is 75% of the original price.
Translating this into an equation, where x is the original price:
       $12 is 75% of the original price
         12 =  0.75 •  x

So you need to solve 12 = 0.75x.

Once you find x, to figure out how much the discount was, you need to subtract 12 from the original price:
      discount = x - 12

I’ve set everything up, you just need to solve 12 = 0.75x