The answer is 0.32mmHg.
A simple way to approach the problem is by using the percentage of CO2 given, which equals the amount of CO2 present in the air sample, as well as its percentage of the total pressure.
According to Dalton's law of partial pressures, the total pressure measured for the sample is equal to
PTOTAL=PCO2+Prest, where Prest represents the pressure the rest of the sample - that is, without the CO2- would have if it occupied the same volume at the same temperature.
Since CO2 makes for 0.042% of the sample, we get
PCO2=PTOTAL⋅0.042100=0.32mmHg, the prssure without the CO2 being 752mmHg−0.32mmHg=751.7mmHg.