You have 4 moles of a gas in a 50 l container held at 2 atm pressure. currently the temperature is 27 ºc. r = 0.0821 l*atm/(mol*k) • what is the temperature in kelvins? k = 27 +273.15 = 300.15 • assuming that everything else remains constant, what will happen to the pressure if the temperature decreases to -15 ºc? the pressure should decrease as well, i believe • if you increase the number of moles to 6 moles, increase temperature to 400k and reduce the volume to 25 l, what will the new pressure be? i have no idea what formula to use

The formula used for determining gas pressure, volume and temperature interaction would be PV=nRT. 

•What is the temperature in Kelvins?
You already right at this part. Kelvin temperature formula from celsius should be:
K= C+273.15= 
K= 27 +273.15 = 300.15 
It is important to remember that the formula in this question is using Kelvin unit at temperature, not Celcius or Fahrenheit.

•Assuming that everything else remains constant, what will happen to the pressure if the temperature decreases to -15 ºC?
In this case, the temperature is decreased from 27C into -15C and you asked the change in the pressure.
Using PV=nRT formula, you can derive that the temperature will be directly related to pressure. If the temperature decreased, the pressure will be decreased too. 

 If you increase the number of moles to 6 moles, increase temperature to 400K and reduce the volume to 25 L, what will the new pressure be?
PV=nRT
P= nRT/V
P= 6 moles* 0.0821 L*atm/(mol*K) * 400K/25L= 7.8816 atm