What is the expression for "the difference between 5 times a number and twice that number"?

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