Use the ideal gas law.
[tex]PV=\text{nRT}[/tex]Where V = 3.5 L, n = 3.4 mol, T = 45 Celsius, and R = 8.31 J/mol*K. But, first, we need to transform the temperature into Kelvin.
[tex]T=45+273.15=318.25K[/tex]Then, find P.
[tex]\begin{gathered} P=\frac{\text{nRT}}{V}=\frac{3.4\text{mol}\cdot8.31\cdot\frac{J}{\text{mol}\cdot K}\cdot318.25K}{3.5L} \\ P=2,569.1atm \end{gathered}[/tex]