How can you minus/subtract as opposed to add?

I am creating an app to log things. For example, I want to say that I bought 100 widgets on the 12th of January 2021 for $100 and on the 15th of January I sold 50 of them for $70.

I can see how to add things, but how do I subtract it and show that action in a log format?

So Page 1/Tab 1 of the app would be:

Buy/Sell Stock Date Price
Buy 100 widgets 12/1/2021 $100
Sell 50 widgets 15/1/2021 $70

And then on the second page/tab it would show how much I made in a graph.

You can do this by:
having two relations on the second tab. 1 relation for what is bien bought and the second one for what’s being sold/

Then, a rollup that sum what you buy and another one to sum what you sell to the respective relations.
lastly have math column to subtract both of the rollups.

I hope this helps/