Basically, this is asking for the value of line CD. When we look at the diagram, we can see two angles at the top of the triangle. They are separated by a completely vertical line. So we know for a fact, that these two triangles are <em>congruent</em>-- they are the same. So line AD, which is 13 units long, must be the same as line CD. Therefore, we can conclude that CD is 13 units long.
