Think backwards for this.
He has 11 dollars at the end of the week.
He gained 9 dollars by washing the car. That's added on money, so you have to subtract that from 11 to get what he got after he went to the movies.
If you subtracted correctly, you should have gotten 2. 2 dollars is half the amount of his allowance. You should know what to do when you encounter half of a whole, but if you don't, you have to multiply what you have (2) by 2 to get one whole since 2 halves equals one whole.
Hope this helped!