How to find TAS for given CAS, FL and OAT?
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