Look for the largest whole number you can factor out of 8 and 18.
This is 2.

Now look for the LARGEST power of x you can factor out.  This is
the SMALLEST power that occurs, but x must appear in ALL terms.
This is 


Now look for the LARGEST power of y you can factor out.  This is
the SMALLEST power that occurs, but y must appear in ALL terms.
This is 

So factor out 



Now the expression in parentheses is the difference of two
perfect squares.  is the same as  and
 is the same as  so that expression in
parentheses factors as the sum of times difference of the bases
of those perfect squares, i.e., their square roots:



Edwin