Yeah thats a common mistake, that even i use to make.
If you go back to basics, when you first did algebra the teacher would usually say that x could be any value. So just for the sake of making it simple, lets just assume x=2 [br][br]18(2)2−12(2)=>72−24=48[br]18(2)2−8=>72−8=64[br] So obviously they do not cancel each other directly, so you couldnt just take out the 18x^2 straight out. However if you took out a factor of 2 out, they could cancel out
In C1, when your doing these sorta questions it can be fairly easy to make mistakes. So i would (if i have time) to substitute an easy x number like 1 or 2 into the original and the simplified version. If they both are the same, then its very likely that your answer is correct . I found myself doing this sort of stuff through to A2 maths which really helped!