Agas at stp changes to a new volume of 955 ml and a new pressure of 6.73 atm. what was the original volume of this gas in ml if the temperature and amount remain constant?

Since the question mentions a change in pressure from 1 atm to 6.73 atm, which triggers a change in volume, then it's premise is Boyle's Law, which notes that pressure is inversely proportional to volume (as volume decreases, pressure increases, and vice versa). As such, Boyle's Equation can be used to determine the initial pressure of the air. Also keen to note is that the Pressure at STP is 1 atm.
Boyle's Equation:      P₁V₁    =    P₂V₂

Since the initial pressure (P₁) is 1 atm, the final pressure (P₂) is 6.73 atm and the final volume is 995 mL, then we can substitute these into the equation to find the initial volume (V₁).

                                                     P₁V₁    =    P₂V₂
∴ by substituting the known values,
                                 ⇒       (1 atm) (V₁ )  =  (6.73 atm)(995 mL)
                                 
                                ⇒                      V₁  =  (6,696.35 atm  mL) ÷ ( 1 atm)
       
                                ⇒                      V₁  =  6,696.35 mL  
                                                                             
Thus, the original volume of this gas in mL is ≈ 6,696.35 mL

Aphysical change is one that you can put in and remove like a ham sandwich. the stuff you put in there can be changem its not permanet. a chemical change is on that can't be change thats permanet. like when you make a cake and bake it. you can't get those ingredients back.