Density = mass / volume
Because the dimensions of the barometer are the same for each case, we can compare the heights of the liquids instead of the volume.
ρ(mercury) = 13.6ρ(water)
mass(mercury)/760 = 13.6 x mass(water)/Height(water)
Masses of the liquids will also be the same.
Height of water = 13.6 x 760
Height of water = 10,336 mm
