A particle moves along a straight line for t seconds after passing through a fixed point O. Its acceleration a [math]ms^{-2}[/math], is given by [math]a=6t-24[/math]. The particle passes through O with a velocity of 36 [math]ms^{-1}[/math]. Find
(a) the range of values of t when the velocity is negative.
(b) the minimum velocity, in [math]ms^{-1}[/math], of the particle