Ed had w more marbles than Doug. Doug lost x of his marbles at the playground. If Ed had y marbles How many marbles does Doug have now?
Let's denote the number of Doug's marbles by d.

d = w + x

Ed's marbles are d - x.

Plugging in for d we get that y = w + x - x.

Therefore, the answer (symbolic expression) is: 

y = w