Consider the free body diagram of the block plane system as shown below.
The expression of the downward acceleration is adown=g‌sin‌α+a‌cos‌α The expression of the upward acceleration is, aup=µ(g‌cos‌α−a‌sin‌α) The condition for the block to remain stationary is, a‌down ‌=a‌up ‌ µ=‌
g‌sin‌α+a‌cos‌α
g‌cos‌α−a‌sin‌α
Substitute the given values in above expression, µ=‌