21 plus what equals 12?

This is how to calculate "21 plus what equals 12?" In other words, 21 plus a number equals 12. What is that number? We start by converting this word problem into an equation where "what" is what we want to solve:

21 + what = 12

To solve this equation, we first add -21 to each side of the equal sign like this:

21 - 21 + what = 12 - 21

Then, we do the calculation to get the answer to "21 plus what equals 12?" as follows:

What = -9

The answer to "21 plus what equals 12?" is -9. You know this answer is correct because 21 plus -9 equals 12.

Plus What Equals Calculator
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