Given 890.0 mmHg what would be the answer in kPa?

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.