Boyles law states that at constant temperature the pressure of gases are inversely proportional to volume of gases.
PV = k
where P - pressure , V - volume and k - constant
P1V1 = P2V2
where parameters for the first instance are on the left side and parameters for the second instance are on the right side of the equation
substituting the values in the equation
7 atm x 4.00 L = 6 atm x V
V = 4.67 L
new volume is A. 4.67 L