combine the like terms so 2/3x +1/2x
 
        
                    
             
        
        
        
The answer is : 3xy(x-2y)
        
                    
             
        
        
        
No
lets say a and b are positive numbers
that means -a and -b are negative
if you make the difference between these to it'd be
-a-(-b) which means -a+b, and if b>a then the difference is positive
        
             
        
        
        
Well if you're wanting to use substitution, you first have to end up with one term on either side of the equation. Use the second one as it's easiest. 
So: x-2y=11, so find -2y as there is a -2y in the first equation. 
then that becomes -2y=11-x. then sub that into equation 1, and you get:
-x+11-x=-13, which equals to -2x+11=-13, which is -2x=-24, so therefore
x=12. then chuck the x into any of the equations to find what y equals. 
hope this helps!