To make the x the subject of an equation simply isolate x. To isolate means to get (something) by itself. To isolate x in this equation, just subtract A from both sides.
Note: Whatever operation you apply to an equation, you must apply to both sides of the equation to keep the sides equal. That is why you must subtract A from both sides.
y = x + A
-A - A
y - A = x
x = y - A
Answer:
x = y - A