To measure the static friction coefficient between a 2.91-kg block and a vertical wall, the setup shown in the drawing is used. A spring (spring constant = 450 N/m) is attached to the block. Someone pushes on the end of the spring in a direction perpendicular to the wall until the block does not slip downward. If the spring in such a setup is compressed by 0.0639 m, what is the coefficient of static friction?