The position of an object moving along x-axis is given by `x = a + bt^2` where `a=8.5 m, b=2.5 m s^-2 and t` is measured in seconds. What is its velocity at `t= 0 s and t = 2.0 s`. What is the average velocity between `t = 2.0 s and t = 4.0 s?`