Answer
$x=0.176, P=59.2torr$
Work Step by Step
We know that according to Raoult's law
$P_{soln}=P^{\circ}x$
We plug in the known values to obtain:
$x_{H_2O}=0.824$
$\implies x_{solute}=1-x_{solvent}$
$x_{solute}=1-0.824=0.176$
Now at $45C^{\circ}$
$P_{H_2O}=0.824(71.9torr)=59.2 torr$
