You said that you have done question (i) so I assume that you won't need to be give all steps so I will explain how to conduct the question.

Displacement is the integral of velocity. For , you will need to integrate to get . You know that when so you will able to find the value of the constant. After the equation of is found, you can substitute to find the displacement at that time.

Same principle but this time we integrate as we want to find and this equation is valid for seconds. You will get a constant so to find it's value, we use the conditions. We know that when . After the equation of is found, you can substitute to find the displacement at that time.

Acceleration is the derivative of velocity. The derivative of give a constant value of implying that for seconds it travelled at the same acceleration so we do not consider that equation. Calculate the derivative of and equate this to , this will lead to one unknown value ( ) which is the variable that you are looking for.