you need to get y by itself on one side of the equation. To do that you need to cancel everything else on that side.
Whatever you do to one side of the equation you have to do to the other
if we add 5 to both sides
y - 5 = 4
+5 +5
-5 plus 5 is 0 so just the y is left on the left side
on the right side we have 4 plus 5 is 9
y=9