Dear student,
At constant pressure, the volume of a fixed amount of a gas is directly proportional to its absolute temperature, this is Charles law.
Mathematically, this can be derived from the ideal gas equation-
PV = nRT
If the amount of gas is fixed which means a number of moles of gas are fixed then n will become constant and R is also a universal gas constant.
So, at constant pressure-
V ∝ T
Regards