The difference in heights of the liquid in the two sides of the manometer is 43.4 cm when the atmospheric pressure is 755 mm hg. given that the density of mercury is 13.6 g/ml, the pressure of the enclosed gas is atm.

The atmospheric P is greater than the P in the flask, since the Hg level is lacking down lower on the side open to the atmosphere. 

43.4 cm x (10 mm / 1 cm) = 435 mm 

the density of Hg is 13.6 / 0.791 = 17.2 times better than the liquid in the manometer. This means that 1 mmHg = 17.2 mm of manometer liquid. 

435 mm manometer liquid x (1 mm Hg / 17.2 mm manometer liquid) = 25.3 mm Hg 

The pressure in the flask is 755 - 25.3 = 729.7 mmHg. 

729.7 mmHg x (1 atm / 760 mmHg ) = 0.960 atm.