Well, torr and Hg are different units, but they have the same conversiton with atm.
The conversion for atm to torr or mmHg is 1 atm = 760 torr = 760 mmHg.
Because you have 6.75 atm you can find both the torr and mmHg.
To use dimensional analysis just make sure that you cancel out your units as you go.
6.75 atm * (760 torr / 1 atm) = 5130 torr or 5.13 x 10 to the -3 torr with significant figures.
I hope this helped.
