#### Answer

$\frac{dp}{dt} = \frac{-1}{2\sqrt {3-t}}$

#### Work Step by Step

$p = \sqrt u$
where,
$u = (3-t)$
Now,
$\frac{dp}{du} =\frac{1}{2\sqrt u} $
and,
$\frac{du}{dt} =-1$
So,
$\frac{dp}{dt} = \frac{dp}{du} \times \frac{du}{dt}$
$\frac{dp}{dt} = \frac{-1}{2\sqrt u}$
$\frac{dp}{dt} = \frac{-1}{2\sqrt (3-t)}$