I dont understand how to solve this problem

A 13-foot ladder is placed against a vertical wall. Suppose the bottom of the ladder slides away from the wall at a constant rate of 4 feet per second. How fast is the top the ladder sliding down the wall when the bottom is 12 feet from the wall? .. . . . 18 The ladder is sliding down the wall at a rate of ft/sec. 5 (Type an integer or a simplified fraction.)