What is the pressure resulting from a gas filling a cubic box with 19.8 m length each side, and filled with 55 moles of gas atoms

Assume that our gas constant is R = 8.315 J/Kmol, and our gas is at room temperature, which is T = 300 K

An astronaut in space cannot use a scale or balance to weigh objects because there is no gravity. but she does have devices to measure distance and time accurately. she knows her own mass is 77.4 kg , but she is unsure of the mass of a large gas canister in the airless rocket. when this canister is approaching her at 3.50 m/s , she pushes against it, which slows it down to 1.30 m/s (but does not reverse it) and gives her a speed of 2.60 m/s . what is the mass of the canister?