The total pressure is the sum of all the partial pressures of the different components in the system. In this case, if the total pressure is 520 torr, and the partial pressure of nitrogen is 266 torr, and the only components are nitrogen and sulfur dioxide, this means that the rest of the pressure is all due to sulfur dioxide. So the partial pressure of sulfur dioxide = 520 - 266 = 254 torr SO2.
