A 6ft tall man is walking toward a lamppost supporting a lig

A 6ft tall man is walking toward a lamppost supporting a light bulb that is 15ft off the ground. At what speed is the man walking at the moment that the length of his shadow is changing at a rate of 4ft/sec? I got -6ft/sec using similar triangles. Please check work. The answer is negative correct (b/c x is decreasing)?

Yes it is coming out to be the same.