Hey there :)
Before translating the sentence into mathematics, let r represent 'a number'
5 times a number = 5r
2 times that number = 2r
The difference between them = 5r - 2r 
* Your equation 
5r - 2r
* Solving the equation 
= 5r - 2r
= 3r
        
             
        
        
        
Answer:
3
Step-by-step explanation:
cause if she writes 4 posts and kevin writes 21 than she only have 3 posts left that she have write
 
        
             
        
        
        
Answer: I gave the evaluate answer and the differentiate w.r.t x answer. Sorry for caps.
Step-by-step explanation: EVALUATE  x4n−3
  DIFFERENTIATE W.R.T. X  (4n−3)x4(n−1)