From the pythagorean theorem, we know that x^2 and y^2 = 15^2, where x and y, where x is the base moving from the wall and y is the distance of the ladder sliding down. This means that y = sqrt(225 - x^2). Plugging in x = 13 gives us y = 2sqrt(14). Differentiating both sides of the original equations gives us dy/dt = 13 x 4 / 2sqrt(14) = 52/2sqrt(14) = 26/sqrt(14) ft/s.
