Let's say the digits of the two digit number is x and y. So the number is xy (x and y are NOT, I repeat NOT being multiplied in this situation). x+y=7. So then yx= xy+9. That means x and y must be one number apart from each other (since when adding 9, the ones digit is decreased by one) and they must equal 7. Well, there isn't really a mathematical way I can solve this but the number is 34 because if the digits are reversed it would be 43 and 43-9=34.