Since the angular acceleration α is constant,
0² - (30 rev/s)² = 2α (60 rev)
Solve for α :
α = -(30 rev/s)²/(120 rev) = -7.5 rev/s²
One revolution corresponds to an angular displacement of 2π radians, so
α = (-7.5 rev/s²) × (2π rad/rev) ≈ -47.12 rad/s²