You simply rearrange the equation so that you get the pressure you want isolated on one side.
The combined gas law equation looks like this
#(PV)/T = "constant"#
You'll use this equation in the form
#(P_1V_1)/T_1 = (P_2V_2)/T_2#, where
#P_1#, #V_1#, #T_1# - the pressure, volume, and temperature of a gas at an initial state;
#P_2#, #V_2#, #T_2# - the pressure, volume, and temperature of that gas at a different state.
So, let's assume you have a gas that goes from
#P_1 = "1.0 atm"#, #V_1 = "5.0 L"#, and #T_1 = "293 K"#, to
#P_2 = "?"#, #V_2 = "3.0 L"#, and #T_2 = "303 K"#
To determine the pressure of the gas under those new conditions, use the combined gas law like this
#(P_1V_1)/T_1 = (P_2V_2)/T_2 => P_2 = T_2/T_1 * V_1/V_2 * P_1#
In this case,
#P_2 = "303 K"/"293 K" * "5.0 L"/"3.0 L" * "1.0 atm" = "1.7 atm"#
