At the core, we are using the INDEX function to retrieve the value at a given row or column number like this:
The range C5:C9 defines the matrix values. What's left is to figure out the correct row and column numbers, and for that we use the MATCH function. To get a row number for INDEX (the impact), we use:
To get a column number for INDEX (the impact), we use:
In both cases, MATCH is set up to perform an exact match. When certainty is "possible" and impact is "major" MATCH calculates row and column numbers as follows:
The INDEX function then returns the value at the fourth row and third column, 12.
For a more detailed explanation of how to use INDEX and MATCH, see this article.