The process used is division. If you go to math is fun.com they have a really good step by step example on how to do this. Please give this brainliest answer if I helped.
        
             
        
        
        
X = -5 is the answer I believe
        
             
        
        
        
If you want the height of the crate it's volume / base area = 48/16 = 3 feet.
        
             
        
        
        
The answer is 
B. 13ab
5ab+9ab-ab
=(5ab+9ab)-ab
14ab-ab
=13ab
Hope it helped : )