A $20\mathrm{.}0-$ft extension ladder is placed against a building. The ladder starts to collapse at a rate of $2\mathrm{.}00$ ft$/$min$.$ The top of the ladder maintains contact with the building while slipping down the building. The bottom of the ladder is not on a banana peel therefore it stays in place at $9-$ft$.$ Find the rate at which the top of the ladder is sliding down the building at the instant the top of the ladder is $12-$ft from the ground.