Answer
$a = \frac{gh}{d}$
Work Step by Step
We can find the required speed at the top of the tube as;
$v^2 = 0+2gh$
$v = \sqrt{2gh}$
We can find the required acceleration in the tube:
$a = \frac{v^2-v_0^2}{2d}$
$a = \frac{(\sqrt{2gh})^2-0}{2d}$
$a = \frac{gh}{d}$
