The missing number is 11.
Step-by-step explanation:



66 - 46 = 20
So what plus 9 will equal 20?

Solve!

And finally....

You're done!
* Of course there is a much easier way to solve this without going through this whole process, which is basically using the commutative property. Since it is addition this will work, but basically changing the order of the numbers won't change the final result. (so the same number had to have been used on both sides)