The only thing you can do is to convert the pressure unit fo kPa.
The conversion factor is:
1 atm = 760.0 mmHg = 101.325 kPa
=> 1 = 101.325kPa / 760.0mmHg
=> 890.0 mmHg * 101.325 kPa / 760.0 mmHg = 118.657 kPa.
With this value you can do any calculation now in kPa.
Also if you have any other value of pressure in mmHg you just use the same factor: 1 = 101.325kPa / 760.0mmHg.