My strategy is to work clockwise around the triangle to find as follows:

First, find since this must be equal to since the length of these two segments is constant:

(note that PQ is a fixed length so its derivative is zero)

Again, if , then since the length of this segment is constant. THIS IS A KEY ASSUMPTION! IS IT CORRECT???? (sorry for the caps)

Then, I find my ultimate answer using Pythagoras:

feet / second

Unfortunately, this answer doesn't check out, so I'm doing something wrong, probably in my key assumption, but I don't see it. Can somebody help?

Thanks.