In order to solve 5 + y = 7 you have to find the value of (y). In this case you could reverse do problem and do 7 - 5 to get the value of y. 7 - 5 = 2, which means y is equal to 2.
5 + y = 7 subtract 5 from both sides
5 - 5 + y = 7 - 5
