Start with the introductory videos to Tableau - Tableau Training and Tutorials
This is a very straight forward question and I suspect you're asking questions before you've learnt the basics on the tool.
the answer is, drag in your dimension (Month) then create a SUM on the Days field.
It looks to me like you need to change your 'Month Invoice Raised' to a dimension, rather than a measure. Also, if you are looking for an average, you'll want to change the aggregation on 'Days Required For Approval' from a sum to an avg.
Now I got my answer ,thanks for the solution.After converting month invoice raised measure to dimension I got my desired result.
Thanks once again.