Block does not move upto a maximum applied force of 3N down the inclined plane. Equation of motion is 3+mg‌sin‌30°=F 3+mg‌sin‌30°=µmg‌cos‌30° .......(i)
Similarly, block also does not move upto a maximum applied force of 12 N up the plane. Now, equation of motion is mg‌sin‌30°+F=12 or, mg‌sin‌30°+µmg‌cos‌30°=12 .......(ii) Solving Eqs. (i) and (ii), we have mg‌sin‌30°=