This problem is dealing with an ideal gas. Given that it is ideal we can proceed the solution using the formula: PV=nRT. The statement says "the voluem does not change", this means that our V is constant. Assuming that the gas is inside a close container the number of moles, n is constant as well. Manipulating the equation where all constants and variables are on different sides:
P/T = nR/V
Solving:
7.58kPa/639 K = x/311K
x = 3.69 kPa