Answer
370 torr
Work Step by Step
$P_{O_{2}}= 0.60\,atm= 0.60\,atm\times\frac{760\,torr}{1\,atm}$
$=456\,torr$
$P_{Ar}=425\, mmHg=425\, mmHg\times\frac{1\,torr}{1\, mmHg}$
$=425\,torr$
$P_{total}=1250\,torr$
By using Dalton's law, we have
$P_{total}= P_{O_{2}}+P_{Ar}+P_{N_{2}}$
$\implies P_{N_{2}}= P_{total}-P_{O_{2}}-P_{Ar}$
$=1250\,torr-456\,torr-425\,torr$
$=369\,torr$
As there are only two significant figures in 0.60 atm, reporting the final answer in two significant figures is better. That is,
$P_{N_{2}}=370\,torr$