Answer: 32-(__)=21
Explanation:
I like to think of what I'm given to be the left side, and what's after to be the right side. If there are 32 originally, then you take some away (since 21 is fewer than 32), that's subtracting by some number. Then, we have 21 left so put 21 on the right side. To solve, subtract the 21 from the 32 and add the unknown over to get (__)=9