Answer
$-319.7\ kJ/mol$
Work Step by Step
Manipulating the given reactions:
$\begin{smallmatrix}
1/4\ P_4(s)+5/2\ Cl_2(g)&\rightarrow&PCl_5(s)&-443.5\ kJ/mol\\
PCl_5(s)&\rightarrow&PCl_3(s)+Cl_2(g)&+123.8\ kJ/mol\\
\hline\\
1/4\ P_4(s)+3/2\ Cl_2(g)&\rightarrow&PCl_3(s)&-319.7\ kJ/mol
\end{smallmatrix}$
