Remember that 1 atm = 760 mm Hg, from there you have 3 methods you could use:
1) Use the "rule of three"
I'm not quite sure how you call that in English, but the concept is simple
1 atm -> 760 mm Hg
0.320 atm -> x mm Hg
From there, multiply in a "cross fashion", i.e.:
1 atm * x mm Hg =760 mm Hg *0.320 atm
x mm Hg =243.2 mm Hg
x = 243.2
2) Use a conversion factor
Since 1 atm = 760 mm Hg is an equality, it means that (1 atm)/(760 mm Hg) and (760 mm Hg)/(1 atm) equals 1, and as such, we can just multiply it by our valor and without changing it.
0.320 atm * (760 mm Hg)/(1 atm) = 243.2 (mm Hg * atm)/(atm) = 243.2 mm Hg
3) Use the conversion formula of linear scales
The conversion formula of all linear scales say "the ratio of the distance between your value and a set value and the distance between two set points is always the same regardless of scale", or, in simpler terms,
(P_a - P_(atm))/(P_(atm) - P_(vac)) = (P_(H) - P_(Hg atm))/(P_(Hg atm)-P_(Hg vac))
Where P_a is the pressure is atm and P_H is the pressure in mm Hg, throwing all values into the formula we have
(0.320 - 1)/(1 - 0) = (P_(H) - 760)/(760 - 0)
-0.680*760 = P_H - 760
-516.8 = P_H - 760
P_H = 243.2