There are several information's already given in the question. Based on those information's the answer can be easily deduced.
Mass of the truck = 1.5 x 10^3 kg
Constant force of resistance = 400 N
Final Velocity = 18 m/s
Initial velocity = 0 m/s
Time taken = 12 s
Then
Acceleration = (Final velocity - initial velocity)/Time
= 18 /12
= 3/2
= 1.5 m/s^2
Then
Driving force = Mass * Acceleration
= 1.5 * 10^3 * 1.5
= 2250 N
So
Net Force = 2250 - 400
= 1850 N
Average power developed = 1850 * 18
= 3.33 * 10^4 W