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"
