Let its initial velocity be v
Let it travel 'l' distance along the incline before it stops,
then using WE theorem
-mv2/2= -mglsin30° - μmgl .... (i)
finally it reaches with speed v/3 using WE again
m(v/3)2/2 = mglsin30° - μmgl .... (ii)
From these two equations you should be able to calculate μ.
im getting μ=0.4