$($Laminar flow$)$ A fluid flows through two horizontal pipes of equal length which are connected together to form a pipe of length $21.$ The flow is laminar and fully developed. The pressure drop for the first pipe is $1\mathrm{.}345$ times greater than it is for the second pipe. If the diameter of the first pipe is $\backslash ($ D $\backslash ),$ determine the diameter of the second pipe.

$\backslash [$

D$\_\{3\}=\backslash$quad D

$\backslash ]$