Step 1 :P1 * V1 = P2 * V2
Step 2 :\(2.6\mathrm{~atm} * 820\mathrm{~mL} = P2 * 2.1\mathrm{~L}\)
Step 3 :\(P2 = \frac{2.6\mathrm{~atm} * 820\mathrm{~mL}}{2.1\mathrm{~L}} * \frac{760\mathrm{~mmHg}}{1\mathrm{~atm}}\)