First, solve for n (number of moles)

Pressure to ATM

Volume to Liters

Temp to Kelvin

PV = nRT

(730.0 mm / 760.0 mm) x (750.0 mL / 1000 mL) = n x .0821 x (20 C + 273 C)

.9605 atm x .75 L = .0821n x 293 K

n = .0299 mol Gas

Then, use same equation for STP

STP = 1 atm, 273 K

PV = nRT

1 atm x V = .0299 mol x .0821 x 273 K

V = .6712 L or 671.2 mL

You can use the combined gas law:

P1V1/T1 = P2V2/T2

Substituting:

(730.0)(750.0)/293 K = (760.0)V2/273 K

Solve for V2

V2 = 671.2 mL

If you use combined gas law, you don't have to convert units (as long as the units for initial and final conditions are the same)