Angular acceleration is given by
[tex]\alpha = 46 t^3 - 11t^4[\tex]
we can write it as
[tex]\frac{dw}{dt} = 46 t^3 - 11 t^4[/tex]
integrating both sides we will have
[tex]w - 0 = 11.5 t^4 - 2.2 t^5[/tex]
now if final angular speed is again ZERO
[tex]0 = 11.5 t^4 - 2.2 t^5[/tex]
[tex]t = 5.23 s[/tex]
So it will be at rest after 5.23 s