The expression of the net acceleration force down the inclined plane is, F=mg‌sin‌θ−µmg‌cos‌θ The expression of net retarding force is, F1=mg‌sin‌θ+µmg‌cos‌θ Given condition, F1=2F Substituting the values, we get mg‌sin‌θ+µmg‌cos‌θ‌‌=2(mg‌sin‌θ−µmg‌cos‌θ) 3µg‌cos‌θ‌‌=mg‌sin‌θ tan‌60∘‌‌=3µ µ‌‌=‌