This is a very common error in mathematics. 

Notice that when there is only MULTIPLICATION under the radical,
what you did would have been correct: 

 is TRUE because




However, when there is ADDITION or SUBTRACTION 
under the radical, that does NOT WORK:  

 is FALSE because



            
So what you did won't work because of the PLUS SIGN 
under the radical.  If they had been MULTIPLIED under 
the radical, then you could have.

If the problem had been this   then the
answer would have been  and then 

However there was ADDITION under the radical, so you cannot
take individual square roots.  All you can do is do whatever 
you can under the radical FIRST.













That's the simplest radical form.

The approximate decimal value is



----------------------

Notice that 



Howver,



Math rules that work when things are MULTIPLIED or 
DIVIDED will NEVER work for things ADDED or 
SUBTRACTED.

and likewise:

Math rules that work for things ADDED or SUBTRACTED 
will NEVER work for things MULTIPLIED or DIVIDED.

Edwin