So I'm going through the book and stumbled on a question that seems blatantly obvious to do, but I keep messing up somewhere. I know Guy-Lusaac's law won't work, and I'm unsure if ln(T2/T1)=ln(P2/P1)^(R/Cp) will work either. Can someone fill me in on the obvious piece I'm missing?

The temperature of Earth's atmosphere drops 5 deg C every 1000m of elevation above Earth's surface. If the ground temperature is 15 deg C with pressure = 760 mmHg, at what elevation is the pressure = 380 mmHg. Assume the air is ideal.