Because they gave us dx dt, it'll actually be more useful to find a relationship between x and theta, and then take the derivative of both sides, and then use this information, possibly, to figure out what the appropriate value of x or theta is at that moment. Let's do that. How does x relate to theta? Well, we use a little bit of trigonometry right over here. If you took the hypotenuse times the cosine of theta, you would get x. Let me write this right over here. x of t, x of t, is equal to the hypotenuse, 20 meters, that's the length of the ladder, times the cosine, cosine of theta, and I could say the cosine of theta of t just to make it clear that this is a function of time.