Related rates, finding an angle.

I was trying to solve this problem:

A ladder 13ft long is leaning against a wall. It suddenly starts to slide away at the base of the ladder. When it is 12ft away from the wall it is sliding away at 5ft/s

if it is 12 feet away then at that moment the other side is 5ft high

I found that at that moment the side on the wall is coming down at -12ft/s

What I can't find is how much the angle between the base and the ground is changing at that moment. I know it is -1 rad/s but I can't get there.