relation rate word problem

2. A ladder 13 ft. long rest against a vertical wall and is sliding down the wall at the rate of 3ft/s at the instant the foot of the ladder is 5 ft. from the base of the wall. At this instant how fast is the foot( or base) of the ladder moving away from the wall? Side note, this problem looks like a pythag. theorem: x^+y^2=r^2. the answer comes out to 7.2 ft./s. though I know what formula to use for the certain situation, I really do not know the steps in solving this problem.

And also, I tried isolating dr/dt and it gave 2x+2y-2r=dr/dt and getting the value for x and y(which i know does not make any sense).