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Suppose the displacement of a particle in motion at time $t$ is given by the parametric equations

$x(t) = \left(2t-1\right)^{2}, \quad y(t) = 6, \quad z(t) = 48t^{4}-32t^{3}.$

(a) Find the speed of the particle when $t = 3$ .
Speed =

(b) Find $t$ when the particle is stationary.
$t$ =

You can earn partial credit on this problem.