U just need to switch ig . Its always the least number first
        
             
        
        
        
The answer is 5. The biggest number - the smallest number (18-13=5)
        
             
        
        
        
Algebra is solving for the unknown so
 if you have 10 apples and you give 5 to your friend, how many apples would you have so that is also 10-5=x solve for x
or it could be more complicated like (x-2)/6+4x-3=17
        
                    
             
        
        
        
The set of integers greater than 50 are (51,52,53,54...)