Weekly Math Problem

This is a missing number problem: What is the missing digit. I don’t want you just to work out the value of ?. I want you to write very clear instructions on how to work it out.

9, 2 ? 1

− 6, 0 7 8

3, 1 9 3

You can share your answers here on Facebook or with me as I visit during the week. Remember you need to show your workings out to get the maximum points.