Is there a way to get the difference between 2 questions?

For example this one? This question is for mongodb. I cant seem to make it work on native query.

metabase

Hi @driggsss
MongoDB has limited support in the GUI, so you would have to use Native query to do such calculation.

Hello @flamber is there any doc for constructing native queries? The most easiest way that I can think of is to have the both questions into 2 separate variables but I'm having trouble making it work or if that is possible

@driggsss It's Mongo native query, the actual JSON. You should check the Mongo documentation. How would you make the query on Mongo directly?
Example:

[
    { 
        "$project": {
            "mydiff": { 
                "$subtract": ["$total", "$subtotal"] 
            }
        }
    },
    {
        "$group": {
            "_id": null,
            "totalDiff": { "$sum": "$mydiff" }
        }
    }
]