Answer:
a = 8
b = -8
Explanation:
You have the following function:

A function is differentiable at a point c, if the derivative of the function in such a point exists. That is, f'(c) exists.
In this case, you need that the function is differentiable for t=2, then, you have:

If the derivative exists for t=2, it is necessary that the previous derivatives are equal:

Furthermore it is necessary that for t=2, both parts of the function are equal:

Then, a = 8, b = -8