Question

In the select part I have too select “+” or “-“

Answer 1

`x--9=x+9`
`x+\text{ -9=x-9}`
`x-+9=x-9`
`x++9=x+9`

This is because we know that

`+*-=-,-*-=+,-*+=-\text{ and +}*+\text{ =+}`