If you are looking for an equation, then that is 2X-8=10. If you are looking for the answer, then X, or the number missing, is 9. Since 9*2=18, and 18-8=10, then the number you are looking for is 9.
If you substitute the 9 back into your equation, it would look like this:
2(9)-8=10 or 18-8=10.
