Total pressure is defined as the sum of all partial pressures in a container, therefore in our question we have:
Ptotal = 5.76 atm
PO2 = 1.31 atm
PN = 2.22 atm
PAr = 0.77 atm
PCO2 = ?
Since the total pressure is the sum of all partial pressures, if we subtract the partial pressures that we know from the total pressure we will have the value of partial pressure for CO2
5.76 - 1.31 = 4.45
4.45 - 2.22 = 2.23
2.23 - 0.77 = 1.46 atm, this is the Partial Pressure for CO2