Jesse's room is w feet wide and x feet long. How much longer is her room than it is wide?
The length of the room is denoted by l.

l = x

The width of the room is denoted by w.

w = w

The difference in length and width is l - w.

Plugging in for l and w we get that the difference is x - w.

Therefore, the answer (symbolic expression) is: 

x - w