Remember volume V and temperature T are directly proportional to each other at constant pressure P and mass n. If you know that T changes by a factor z, then you can calculate Vfinal = z x Vinitial
For example, if the temperature of a 3.5 L sample of helium gas is increased from 300 K to 900 K, what is its new volume?
The temperature has increased by a factor of 3 (900/300) so:
Vfinal = z x Vinitial
Vfinal = 3 x 3.5 L
Vfinal = 10.5 L
Be sure the temperature is expressed in Kelvin. If you are given a temperature in degrees Celsius or Fahrenheit, convert it to Kelvin before determining the factor z.