Answer
(a) $34mJ$
(b) $2.1\mu Pa$
Work Step by Step
(a) The required amount of energy can be determined as
$E=P_{avg}\Delta t$
We plug in the known values to obtain:
$E=(2.8\times 10^{-3}W)(12s)$
$E=33.6\times 10^{-3}J$
$E=34mJ$
(b) We can find the required pressure as follows:
$p=\frac{P_{avg}}{Ac}$
We plug in the known values to obtain:
$p=\frac{2.8\times 10^{-3}W}{(\pi(\frac{2.4\times 10^{-3}m}{2})^2)(3\times 10^8m/s)}$
$p=2.1\times 10^{-6}Pa$
$p=2.1\mu Pa$
