The sum of a number and three less than the number is sixteen. how do you write that in numbers

Its: 

Start at the beginning
The sum of a number is  and three less than the number is 
Parentheses are mandatory; it is asking the number you get from 
Its asking for the sum so you would get 
In the very large dictionary of mathmatical terms, in this kind of problem, the word "is" means =.
The problem says "is sixteen"
So you would get 

I’m not sure how this works