An 8-foot ladder is leaning against a wall. The top of the ladder slicing down the wall at the rate of 2 feet per second.

An 8-foot ladder is leaning against a wall. The top of the ladder slicing down the wall at the rate of 2 feet per second. How fast is the bottom of the ladder moving along the ground at the point in time when the bottom of the ladder is 4 feet from the wall? 

Explain your answer and draw a picture for this question.