equations, of the line. 
<div class="review-answer">Answer: A vector
    equation is $\vec r(t) = (4,5,6)t+(1,2,3)$ or $\vec r(t) =
    (4t+1, 5t+2, 6t+3)$. Parametric equations for this line are
    $x=4t+1$, $y=5t+2$, and $z=6t+3$.</div>
  </div>

  <div class="problem">
    Consider the parametric surface $\vec r(a,t) = (a\cos t, a\sin
    t, t)$ for $2\leq a\leq 4$ and $0\leq t\leq 4\pi$. We
    encountered this parametric surface in chapter 5 when we
    considered a smoke screen left by multiple jets.