I dont get how has the pressure been balanced in this question?

see as the temperature is fixed therefore the ideal gas equation apllies here.
we know that
PV=nRT
Here temperature is constant therefore nRT will be constant as number of mole of gas enclosed in the mercury column is constant
therefore
PV(horizontal condition)=PV(vertically)
for horizontal ,P=76cm =1 atm,V=cross sectional are*length=a*10
for vertical,P=84cm,V=crosssectional area*length =a*l2
l2=9.05cm
here there is confusion as the length of the air column that is being shown here is wrong .It should be form the bottom to 8cm that is l2