... r + 7 = 9
... r
Subtract 7 from both sides of the equation.
... r + 7 - 7 = 9 - 7
... r = 2 . . . . . . simplify
Expressed as a solution set, this may look like ...
... r ∈ {2}
_____
You probably had no trouble solving this in 2nd grade when it was shown to you in the form ...
__ + 7 = 9