How do I update a list item after adding it with Power Automate

I can't figure out how to do a very simple update of a column value in Power Automate in SharePoint online. I've seen examples of how to update an item on another list, but nothing that updates a value on the same item that was just added and triggered the workflow. Can anybody give me an example and maybe a very brief explanation?

I want to multiply the values of two columns and add it to a third column on the same item which will be blank. I used the formula to update the column I want using values from other columns. =[Dollar Amount]*[% Chance] but it says invalid syntax. There seems to be nothing intuitive in this new power automation and it's a horrible failure compared to how easy it used to be. MS is always making these mistakes.