Calculating difference between Average(Weightd_Avg) and Average(Previous_WeightedAvg) in Subtotals

I am creating a PowerBI Report with 3 number columns which basically has Weighted Average, Previous_WeightedAvg and Delta.  I have calculated all these 3 in SQL. The weighted average and previous_weightedAvg is calculated based on the rating and weight for each NIST category and subcategory against Microsoft's various Services when the service was refresh currently and the last time. The rating could be "1,2,3,4,NA"

Currently this report is being generated manually in excel. Please find below the view:

 Asset Management is the category and it has 6 subcategories. Eac of them have respective values for Weighted_Avg, Previous_WeightedAvg and Delta (Weighted_Avg - PreviousWeighted_Avg). The number in color blue is the Average of (Weighted_Avg), number in green is the Average of (Previous_WeightedAvg). I can obtain these numbers easily in PowerBI by selecting Average and Subtotals. However the number in yellow is not the average of (Delta). It is the difference between the number in blue and number in green. 

I have to create same view in PowerBI matrix. It is possible to have Average of numbers in 2 columns and difference in the third column  (Delta) at Subtotal level?

Please help!

Thanks,

Nancy