Here is how I think about this problem.
Say a function like this X = Y
We want to keep the left side of the value equal to the right side value at all time.
So here it is:
If you want to add 7 on the left side like this
X+7, X+7 does not equal to Y because X = Y
In order to make this true, you will have to make sure you counter the effect on the Y side so you minus 7 like this Y - 7
X+7 = Y - 7 the +7 and -7 counter each other to 0.
So lets say X-7 = Y
I want to get ride of 7 on the left side of the equation, what do I do?
I need to add value 7 on the ride side to counter the loss of the left side
Therefore it is
X = Y + 7.