345 ml of hydrogen gas is collected over water at 40 degrees c where pressure is 765 torr.

what is the volume of the dry gas at 40 degrees c and STP?

You need to know the vapor pressure of water at 40 °C. The source below says 55.324 torr.
So the pressure of the dry gas is 765 - 55.324 = 710 torr.

At STP the dry gas would occupy:
(345 mL) x (710 / 760) x (273 / (40 + 273)) = 281 mL