STP conditions imply a pressure of 100 kPa and a temperature of 273.15 K. Under these conditions, * 1mole* of any ideal gas occupies exctly 22.7 L - this is known as the molar volume of a gas at STP.
So, in order to determine the volume of the sample at STP, all you really need to know is how many moles of gas you're dealing with.
To do that, use the ideal gas law equation for the initial conditions of the sample.
This means that the new volume of the sample will be
Rounded to two sig figs, the answer will be
SIDE NOTE Many online sources and textbooks still list the old STP conditions. more precisely 1 atm and 273.15 K, which makes the molar volume of a gas equal to 22.4 L.
If you are supposed to use the old value of the molar volume of a gas at STP, simple replace 22.7 with 22.4 in the final calculation.