Can you help me with an excel question-what if analysis-goal seek?

Not sure how to use this feature trying to find how many procedures per day it will take to break even.

What do I plug into excel to use this feature?

I know from any cell click on what if analysis, then goal seek

Set cell ???

Set to value to 0

By changing cell ???

I know it has to be between 18-20 procedures per day because the base is 20 procedures per day, making $875,020 or changing %-10 from the base case to 18 revenues becomes (-85,604).

trying to find how many procedures it will take to break even