Your variables, $"L"$ and $""^@"C"$ represent volume and temperature. The gas law that involves the relationship between the volume of a gas and its temperature is .
Charles' law states that the volume of a given amount of gas held at constant pressure is directly proportional to its Kelvin temperature. This means that as the temperature increases, the volume also increases, and vice-versa. The equation used to solve gas problems involving Charles' law is:
$V_1/T_1=V_2/T_2$,
where the temperatures must be in Kelvins. Add $273.15$ to the Celsius temperature to get the Kelvin temperature.
Organize your data:
Known
$V_1="8.62 L"$
$T_1="221"^@"C" + 273.15="494 K"$
$V_2="2.61 L"$
Unknown
$T_2=?$
Solution
Rearrange the equation given above to isolate $"T_2$. Insert your data into the new equation and solve.
$T_2=(V_2T_1)/V_1$
$T_2=(2.61color(red)cancel(color(black)("L"))xx494"K")/(8.62color(red)cancel(color(black)("L")))="150. K"$
To convert from Kelvins to degrees Celsius, subtract $273.15$ from the Kelvin temperature.
$"150. K"-273.15"="-123^@"C"$