Substitute a number for x in the original expression
and then substitute it in the final answer and see if 
you get the same result.

First substitute  in the original problem:







You could have done that in your head!

Now substitute  in your final answer:






They're the same!  Both !
You could have done that in your head, too.
That's not a perfect check, but it is good
evidence that it is correct.  [However, if 
you hadn't have gotten the same result you 
would have known for certain that your
answer would have been wrong!]

To be much surer, also substitute  
in the original expression and also in the 
final answer. 

Substitute  in the original problem:








Now substitute  in your final answer:






They're the same!  Both !

When both x=1 and x=0 give the same results when
substituted in the original and in the final answer,
then it is extremely likely that your answer is 
correct.  You can also substitute  or any 
other number you choose if you want to be absolutely 
sure!

Edwin