Calculate the pressure of the gas samples as indicated by the manometer
In part (A), ${\rm P}_{\rm gas} &gt; {\rm P}_{\rm atm}$, as evidenced by the column of mercury being "pushed up" in the manometer on the side of ${\rm P}_{\rm atm}$ - in this case, you'll need to add (the amount it's "pushed up") to atmospheric pressure (i.e., 764 mm Hg). You state that you tried to subtract 40.0 from 764.0: Look at the graphic again (I hold it's 7 cm - or 70 mm - Hg) and try adding that number to 764 mm Hg instead of 40 mm Hg.

For part (B), ${\rm P}_{\rm atm} &gt; {\rm P}_{\rm gas}$, so reverse the process above. I believe what you state is your answer to part (A) is actually the correct answer to part (B).