Answer:
6 s
Step-by-step explanation:
Let angular acceleration=

Rotational mass=I
Time,t=2 s

Angular acceleration,
...(1)
...(2)
Equation(2) divided by equation (1)



When alpha decreases by 1/3 factor then time increases by 3
The motor needs 2 s originally.If the rotational mass is 3 times the original rotational mass then it takes 3 times the original time.
Time=

Hence, the motor need to bring the blade to its full angular velocity=6 s