We first consider the gases that will be present in that sample.
First, there will be nitrogen, as stated. Second, there will also be water in the form of water vapor. For this, we need the vapor pressure of water at 23.0 °C, which is about 21.0 mmHg. Now, the sum of the vapor pressures of the gases will be equivalent to the total pressure. So the pressure of nitrogen gas is:
785 - 21
= 764 mmHg