What's the coefficient of static friction?

A block sits at rest on the tilted track. We tilt the track farther and farther, until the block JUST ABOUT starts to slide. This happens at a critical angle θ.

Draw a set of coordinate axes which are tilted to match the track.

List all components of each force.

Write down equations for total forces in X-direction, and total forces in Y-direction. Both should add up to zero.

What is the coefficient of static friction between the block and the ramp?