How to find TAS for given CAS, FL and OAT? Here is another question for my PPL Exam. If anyone can illuminate me with explaining every step to achieve TAS with given parameters, it would be great. 

Given:
Calibrated airspeed (CAS): 155 kt. Flight level (FL) 80.
Outside air temperature (OAT): -15° C.
The true airspeed (TAS) equals... 

When seeing this question there is only one thing is coming up to my mind and which is: Temperature drops 2° C every 1000 ft but I cannot connect the dots to find TAS.
My trusty E6-B flight computer (I can't believe we still teach people to use what is essentially a slide rule) has the following printed on it:

AIRSPEED CORRECTION
Set PRESS. ALT opposite °C in window. Opposite CAS on inner scale read TAS on outer scale. Read Density ALT Centre ▲

Following the instructions and setting -15°C against 8,000 ft reads a TAS of 170 kts for a CAS of 155 kts