The question is asking you to get X on its own using algebra techniques
In this case the first step is to take b away from both sides then divide by 3
a-b = 3x
(a-b)/3 = x
a = 3x + b
put -b on both sides then divide by 3 on both sides
a - b / 3 = 3x +b -b / 3 we get
(a-b) / 3 = x
this is for x