Here the pressure difference between the sea level and the hill and that due to atmospheric pressure would be same.
So,
Pressure difference between sea level and the hill would be
dP1 = dh1 X ρm X g (1)
where dh = h1 (sea level mercury height = 75 cm) - h2 (hill top mercury height = 50 cm)
ρm is the density of mercury
and the pressure difference in atmospheric pressure would be
dP2 = dh2 X ρa X g (2)
where dh2 is the height of the hill, ρa is the density of air
now, we know that (1) = (2), so
dh1 X ρm X g = dh2 X ρa X g
by substituting appropriate values and rearranging, we get
dh2 = 0.25 X (ρm/ ρa) = 0.25 X 104 m
or dh2 = 2500 m
thus, the height of the hill is
dh2 = 2500m = 2.5 km