Two Ladders and a Narrow Street

Topic:
Algebra
You are living on a narrow street with buildings on each side. Two ladders are placed with their bases resting against opposite walls. Each ladder is then leaned across the street to rest on the opposite building. You measure from where the two ladders cross directly down to the street (the height of the intersection).
If you know the lengths of the ladders and the height of the intersection, how can you find the width of the street?