Answer:
a) 10^6 m/s^2
b) 7.39 s
Step-by-step explanation:
Hello!
If the velocity is given by:

Then the acceleration is the derivative of the velocity with respect of time:

a)
Therefore the acceleration at t=5.0s is:

b)
The velocity will become unphysical when it equals the velocity of ligth (3x10^8 m/s)
3x10^8 m/s = 2x10^5 m/s + 10^5 m/s^3 t^2
Solving for t:
t^2 = 2998 s^2
t = 7.39 s