Answer
The IV bag must be hung at least 15.5 cm above the vein.
Work Step by Step
We can find the height $h_f$ of the fluid that would have the same gauge pressure as 12 mm Hg:
$\rho_f~g~h_f = \rho_m~g~h_m$
$h_f = \frac{\rho_m~h_m}{\rho_f}$
$h_f = \frac{(13.6~g/cm^3)~(12~mm)}{1.05~g/cm^3}$
$h_f = 155~mm = 15.5~cm$
The IV bag must be hung at least 15.5 cm above the vein.
