Use the ideal gas law. The formula is #PV=nRT#, where #P# is pressure, #V# is volume, #n# is moles, #R# is the gas constant, and #T# is the Kelvin temperature.
Given/Known
#P="750 mmHg"="750 Torr"#
#V=890cancel"mL"xx(1"L")/(1000cancel"mL")="0.89 L"#
#R="62.363577 L Torr K"^(-1) "mol"^(-1)#
https://en.m.wikipedia.org/wiki/Gas_constant
#T="21"^@"C"+273.15"=294 K"#
Unknown
#n#
Solution
Rearrange the formula to isolate #n#. Substitute the given/known values into the formula and solve.
#PV=nRT#
#n=(PV)/(RT)#
#n=(750cancel"Torr" * 0.89cancel"L")/((62.363577 cancel"L" cancel"Torr" cancel"K"^(-1)" mol"^(-1))xx(294cancel"K"))="0.036 mol"# rounded to two significant figures
