Boyle's law gives the relationship between pressure of a gas and volume. It states that the pressure of a gas is inversely proportional to volume of gas at a constant temperature. PV = k where P - pressure , V - volume and k - constant P1V1 = P2V2 where parameters for the first instance are on the left side of the equation and parameters for the second instance are on the right side of the equation. 1.50 atm x 500.0 mL = P x 150.0 mL P = 5 atm applied pressure should be 5 atm to compress the gas.
