Phil received a prize of x dollars from a poker tournament. The tournament cost him 100 dollars to enter. What were Phil's net winnings from the tournament? Write your answer as an expression.

Phil gets x dollars, but before he gets x dollars, he needs to pay 100 dollars to enter the tournament. Therefore the net winning is not x dollars but you should substract 100 dollars from x.
So, the expression of the net winning will be x-100