At constant pressure, temperature is directly proportional to the volume and vise versa. The formula will be
V1/T1 = V2/T2
where V1 and T1 are the initial volume and temperature and V2 and T2 are the final volume and temperature. The temperature is in Kelvin and to convert Celsius to Kelvin add 273.
so, 6.24L/298 = V2/328
=6.87L