Now let me draw our 20 meter ladder. Maybe it looks something like that. That is 20 meters. They say the distance x of t between the bottom of the ladder and the wall, so it's this distance right over here, this distance right over here is x of t. They say it's increasing at a rate of three meters per minute. We know that we could either say x prime of t, which is the same thing as dx dt, is equal to three meters. I'll write it out, because it's hard if I just said m per m. It might not be that clear. Meters per minute, so they give us that piece of information.