At an atmospheric pressure of 870 mm hg of 21% oxygen, the partial pressure of oxygen is

Atmosphere is made of a mixture of gases. Each individual gas exerts a pressure known as the partial pressure.
The sum of the partial pressures exerted by each gas in the mixture is equal to the total pressure exerted by the mixture of gases as a whole.
The partial pressure depends on the composition of the gas in the mixture.
In this case the total pressure - 870 mmHg
21% of atmosphere is oxygen
partial pressure of Oxygen = 870 mmHg x 21%  = 182.7 mmHg