A 2.0-{rm kg} box rests on a plank that is inclined at an angle of 65 ^circ above the horizontal. The upper end of the box is attached to a spring with a force constant of 390 N/m, as shown in the figure .
If the coefficient of static friction between the box and the plank is 0.21, what is the maximum amount the spring can be stretched and the box remain at rest?