Agas originally at 27 °c and 1.00 atm pressure in a 2.6 l flask is cooled at constant pressure until the temperature is 11 °c. the new volume of the gas is

Charles law gives the relationship between volume and temperature of gas at constant pressure 
it states that at constant pressure, volume of gas is directly proportional to temperature 
V/T = k
where V - volume T - temperature and k - constant 

parameters for the first instance are on the left side of the equation and parameters for the second instance are on the right side of the equation
T1 - temperature in Kelvin - 27 °C + 273 = 300 K
T2 - 11 °C + 273 = 284 K
substituting the values in the equation 
2.6 L / 300 K = V / 284 K 
V = 2.46 L
New volume of the gas is 2.46 L