the above question is simply based on simple harmonic motion as the force acting on the liquid is acting in opposite direction to position of the liquid level and the force is proportional to the displacement of the fluid.
so,
f=2pAxg x is instaneous distance of the liquid level from mean...
The answer is in your question .Since the truck is deaccelerating from 70mileshr-1 to 0mileshr-1 .Since this change is taking place in time 't' so a=70/t.
Practically this time will be very small (i.e in the order of 1*10^-1) so this force must be very large (i.e a=70/10^-4=7x10^-5 , assuming...