You have to use the Ideal Gas Law equation for this equation. The Ideal Gas Law equation is:
PV = nRT
P = the pressure (in atm)
V = the volume in L
n = the number of moles of gas
R = Gas Constant (for this problem it is 0.082057 L*atm/(mol*K) )
T = temperature in Kelvin
The problem asks us to find the pressure, so P = what we want to find. The problem says "18.0 L container" so V = 18.0 L. The problem says that there are "1.20 moles of oxygen gas" so n = 1.20. The temperature must be in Kelvin, but the question gives us a temperature of 45.0 degrees C. We convert Celsius to Kelvin using the following conversion: K = 273.15 + Celsius. So K = 273.15 + 45.0, Thus, the temperature is 318.15 Kelvin. So T = 318.15 K.
If we put all this information in the Ideal Gas Law Equation, it looks like this. We solve for P:
PV = nRT
P * 18.0 = 1.20 * 0.082057 * 318.15
P * 18.0 = 31.3277215
P = 31.3277215 / 18.0
P = 1.74 atm.
So the pressure is 1.74 atm.