Let's see if I remember this...
a) You are determining the amount of normal (upward) force that must be applied in order to counter the gravitational force of the helicopter. You may use the Fnet equation in order to solve this (note how Fnet = 0 as the helicopter is not accelerating if it is staying still in the air).
Fnet = Fg + F(up)
0 = Fg + F(up)
0 = mg + F(up)
0 = 1500 kg(9.8 N/kg [down]) + F(up)
-F(up) = 1500 kg(9.8 N/kg [down])
-F(up) = 14700 [down]
F(up) = 14700 N [up]
Therefore, the amount of upward force required to keep the helicopter at a constant height is 15000 N [up]. (Using significant digit rules).
b) You are determining the acceleration of the net force to figure out the maximum upward acceleration of the helicopter. You may use the Fnet equation in order to solve this.
Fnet = Fg + F(up)
ma = mg + F(up)
1500kg(a) = 1500kg(9.8 N/kg [down]) + 20000N [up]
1500kg(a) = 14700 N [down] + 20000N [up]
1500kg(a) = 14700 N [down] - 20000N [down]
1500kg(a) = -5300 N [down]
1500kg(a) = 5300 N [up]
a = 0.28301 m/s² [up]
Therefore, the maximum upwards acceleration of the helicopter would be 0.3 m/s² [up]. I am not too sure if I have done significant digit rules correctly for this question, but I hope I helped!