#### Answer

172 mmHg

#### Work Step by Step

$V_{1}=3.50\,L$
$P_{1}=735\, mmHg$
$V_{2}=15.0\,L$
Temperature and amount of gas are constant.
Boyle's law states that
$P_{1}V_{1}=P_{2}V_{2}$ (unchanging T and n)
$\implies P_{2}=\frac{P_{1}V_{1}}{V_{2}}=\frac{735\, mmHg\times3.50\,L}{15.0\,L}$
$=172\, mmHg$