The pressure of air in a 2.25 l container is 1.20 atm. what is the new pressure if the sample is transferred to a 6.50 l container? temperature is constant.

Boyle's law gives the relationship between pressure of gas and volume.
It states that for a fixed amount of gas at constant temperature, pressure of gas is inversely proportional to volume.
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 given on the right side of the equation.
substituting the values in the equation;
1.20 atm x 2.25 L = P x 6.50 L
P = 0.415 atm
new pressure is 0.415 atm