how do i do this step by step how do i know which is a b and c

. A 15-ft ladder Use Desmos Prob 3 is leaning against a wall on a flat terrain. The base of the ladder is sliding away from the wall. By the time the base is 12 ft from the house, it is moving horizontally at the rate of 3ft/sec. (Keep in mind that the top of the ladder is in contact with the wall at all times.) (a) How fast is the top of the ladder sliding down the wall when the base is 12 ft from the wall? (Remark: this part is the same as an example in the book except that numbers are changed.)