Answer: C
Explanation:
You are trying to find the number of muffins Nola started the day with.
When she baked 21 muffins she gained 21 muffins (addition)
When she gave away 30 muffins she lost 30 muffins (subtraction)
The third equation has her gaining 21 muffins and losing 30 muffins for a final amount of 44 so it must be C