Ideal gas law: PV=nRT where P is pressure, V is volumes, n is amount of substance of gas, R is the ideal gas constant, and T is temperature. Measured in SI units, n is measured in moles, T in kelvin, and R has a value of 8.314 J/(K·mol).
Since you are dealing with only volume and pressure and the amount of the substance and temperature remain the same, P₁V₁=nRT and P₂V₂=nRT, thus P₁V₁=P₂V₂.
Plug in your givens. The gas initially has a volume of 22.4 mL and a pressure of 1.21 atm. After the decrease, it has a pressure of 82 mmHg. So, V₂ is the unknown variable.
(1.21 atm)(22.4 mL)=(82 mmHg)V₂
Another problem arises. The units of pressure on the left don't match the units of pressure on the right, so you have to convert either one to match the other. I suggest converting mmHg to atm. 82 mmHg * (1 atm/(759.99 mmHg)) = .11 atm
Plug that in for 82 mmHg.
(1.21 atm)(22.4 mL)=(.11 atm)V₂
Solve for V₂. Divide by .11 atm.
(1.21 atm)(22.4 mL)/(.11 atm)=V₂
The reason I use the ideal gas law instead of boyle's law is because you can easily remove whichever variables from the equation that don't change. For example, P and V may remain constant but n and T might change. You don't have to remember multiple equations, just this one.