You have to compare the of $"CO"_2$ produced experimentally at STP to the accepted density of $"CO"_2$ at STP.
$"density"="mass"/"volume"$
You are given the mass and volume of the experimentally produced $"CO"_2:$$"mass"="0.10 g";$$"volume"="0.056 L"$
Insert your data into the density formula and solve.
$"density CO"_2"="(0.10"g CO"_2)/(0.056"L CO"_2)="1.8 g/L"$
$"Percent error"=abs(("experimental value"-"accepted value")/("accepted value"))xx100$
Insert your data into the percent error equation.
$"Percent error"=abs(("1.8 g/L"-"1.96 g/L")/("1.96 g/L"))xx100="8.2%$ (rounded to two significant figures)