Partial pressures follow the following rules: sum of Pi = PTot Pi = xiPTot where xi is the mole fraction of i (ni/ntot) and you can use IGL or CGL independently for each gas so that PiVTot = niRT
So in this problem you know that PHyd = PTot - POxy - PNit
This is actually more of a conversion problem because you have to put all the pressures in the same units. Use standard atmospheric pressure to relate the units: 101.3 kPa = 1 atm = 760 mmHg.