since circle b has a radius of 5 and radius is have of the diameter you have to do 5+5 to get the diameter when you do that you get 10 and then you have to subtract circle a diameter which is 9 from circle b diameter which is 10 so 10-9 is 1
You should be thinking about a negative number. In fact, you should be thinking about a number that is the opposite of 5! -5! So, the additive inverse of 5 is -5
