The parameters a and b are specific to the gas in question. The parameter a accounts for intermolecular attractions between gas particles, and the parameter b accounts for the volume of the gas molecules. Like any equation, you could calculate the value of one of the constants given all other values, but usually they're just given to you.
The answer should be more accurate, because the ideal gas law assumes the gas you are examining is an ideal gas, meaning no intermolecular forces and particles have no volume. The parameters a and b explicitly account for this.