We can't subtract because they don't have the same 
denominator.  The LCD of 9 and 6 is 18, because 
both will divide evenly into 18. So we change 
to  and  to 



We still can't subtract because the upper fraction  
has a smaller numerator than the lower fraction .
So we borrow 1 from the 9, leaving it with 8, and that gives 
us  to add to the fraction  which just amounts 
to adding the denominator 18 to the numerator 10, getting 28, so 
we have this:

 

And now we can subtract because the numerator above is bigger
than the numerator below. So  8 minus 6 gives 2 for the whole part
of the answer, and 28-15 gives 13 for the numerator of the answer
so we subtract and get:

 

Edwin