For this question we will be using Gay-Lussac's Law, which tells us that the pressure of a gas increases as its temperature increases, if we have a constant mass and volume. The formula for this law is:
P1/T1 = P2/T2
We have:
P1 = 5 atm
T1 = 300 K
P2 = ?
T2 = 900 K
Now we add the values in the formula:
5/300 = P2/900
P2 = approximately 15 atm will be the new pressure, since we have an increase of temperature of times 3, pressure will also increase times 3