Because the bird hit the clock tower which made the bell ring that made the cups shake and fall onto little tommy's hand and made the ice cream cone fall on the ground. Then he got hit by a bus.
running a perpendicular line like so from the right-angle, makes three similar triangles, the large, containing the smaller ones, and a medium and a small one. Now, using proportions, you can just use there, the medium and smalle to get "x".