Question

What are the steps in finding the velocity and acceleration functions when x(t)=6

Then how do I find the velocity and acceleration at 2.5s?

Answer 1

Very simple. Here your particle always stays at the same position, 6, for all values of t. Your particle is standing still. v(t)=0 and a(t)=0 for all t.

Answer 2

In general, to find the velocity function, take the first derivative of the position function with respect to time. The acceleration is the second derivative of the position function with respect to time (or the first derivative of the velocity function).